diff --git a/src/game/battle/battle.ts b/src/game/battle/battle.ts
index 73b91c5..7638005 100644
--- a/src/game/battle/battle.ts
+++ b/src/game/battle/battle.ts
@@ -908,7 +908,37 @@ export class Battle {
     };
     this.player.stats = mon.stats.slice(1);
     this.applyStatusEffects(this.player);
-    // No badges exist yet, so BadgeStatBoosts changes nothing.
+    this.badgeStatBoosts();
+  }
+  /** BadgeStatBoosts: Johto badges raise the battle Pokémon's Attack,
+   * Defense, Speed and Special Attack by an eighth (to at most 999). The
+   * PlainBadge and MineralBadge bits are swapped first. The Glacier Badge's
+   * Special Defense boost tests the register that BoostStat leaves after
+   * Special Attack, so it depends on that stat (a kept bug). */
+  private badgeStatBoosts() {
+    const badges = this.options.johtoBadges ?? 0;
+    // ZEPHYR, HIVE, FOG, STORM, GLACIER and RISING stay; PLAIN (bit 2)
+    // moves to bit 4 and MINERAL (bit 4) to bit 2.
+    let b = (badges & 0xeb) | ((badges & 0x04) << 2) | ((badges & 0x10) >> 2);
+    const stats = this.player.mon.stats;
+    // BoostStat, returning the A register it leaves.
+    const boost = (index: number) => {
+      const value = stats[index] + (stats[index] >> 3);
+      if (value >= C.MAX_STAT_VALUE) {
+        stats[index] = C.MAX_STAT_VALUE;
+        return C.MAX_STAT_VALUE & 0xff;
+      }
+      stats[index] = value;
+      return ((value >> 8) - (C.MAX_STAT_VALUE >> 8) - ((value & 0xff) < (C.MAX_STAT_VALUE & 0xff) ? 1 : 0)) & 0xff;
+    };
+    let a = 0;
+    for (let index = 0; index < 4; index++) {
+      a = b;
+      const carry = b & 1;
+      b >>= 2;
+      if (carry) a = boost(index);
+    }
+    if (a & 1) boost(4);
   }
   /** ApplyPrzEffectOnSpeed and ApplyBrnEffectOnAttack for one side. */
   private applyStatusEffects(side: Side) {
@@ -1599,6 +1629,7 @@ export class Battle {
       const value = Math.floor((player.stats[stat] * numerator) / denominator);
       player.mon.stats[stat] = value === 0 ? 1 : Math.min(999, value);
     }
+    this.badgeStatBoosts();
     this.applyStatusEffects(this.turn === 0 ? this.player : this.enemy);
   }
   /** PrintPartyMenuActionText with the Pokémon's nickname. */
@@ -4985,13 +5016,15 @@ export class Battle {
     }
     this.failedMessage = 0;
   }
-  /** CalcPlayerStats / CalcEnemyStats: stages, then status penalties. */
+  /** CalcPlayerStats / CalcEnemyStats: stages, the player's badge
+   * boosts, then status penalties. */
   private calcStats(side: Side) {
     for (let stat = 0; stat < 5; stat++) {
       const [numerator, denominator] = this.data.statMultiplier(side.levels[stat]);
       const value = Math.floor((side.stats[stat] * numerator) / denominator);
       side.mon.stats[stat] = value === 0 ? 1 : Math.min(999, value);
     }
+    if (side === this.player) this.badgeStatBoosts();
     this.applyStatusEffects(side);
   }
   private statName() {
@@ -6019,8 +6052,13 @@ export class Battle {
         side.mon.hp = mon.hp;
         side.mon.maxHp = mon.stats[0];
         side.mon.level = newLevel;
-        side.stats = mon.stats.slice(1);
-        this.calcStats(side);
+        // A transformed Pokémon keeps its copied stats.
+        if (!side.has(SUB5, C.SUBSTATUS_TRANSFORMED)) side.stats = mon.stats.slice(1);
+        // ApplyStatLevelMultiplierOnAllStats, ApplyStatusEffectOnPlayerStats,
+        // then BadgeStatBoosts.
+        this.applyStatLevelMultipliers(side);
+        this.applyStatusEffects(side);
+        this.badgeStatBoosts();
       }
       // LevelUpHappinessMod: more where it was caught.
       this.changeHappiness(
diff --git a/src/game/battle/constants.ts b/src/game/battle/constants.ts
index def3b2e..61ba1d6 100644
--- a/src/game/battle/constants.ts
+++ b/src/game/battle/constants.ts
@@ -249,6 +249,7 @@ export const C = {
   PP_MASK: 63,
   NUM_MOVES: 4,
   BASE_HAPPINESS: 70,
+  MAX_STAT_VALUE: 999,
   START_MONEY: 3000,
   MAX_MONEY: 999999,
   MAX_COINS: 9999,
diff --git a/tests/reference-badges.spec.ts b/tests/reference-badges.spec.ts
new file mode 100644
index 0000000..eeea40f
--- /dev/null
+++ b/tests/reference-badges.spec.ts
@@ -0,0 +1,69 @@
+import { test, expect } from "@playwright/test";
+import { existsSync } from "node:fs";
+import { romPath, symbolsPath, loadSymbols, openReference } from "./reference-fixture";
+
+test.skip(!existsSync(romPath) || !existsSync(symbolsPath), "Needs private ROM and reference symbols.");
+test("badge stat boosts match the original for every Johto badge byte", async ({ page }) => {
+  const { at, routine } = loadSymbols();
+  const addresses = {
+    romBank: at("hROMBank"),
+    badges: at("wJohtoBadges"),
+    linkMode: at("wLinkMode"),
+    tower: at("wInBattleTowerBattle"),
+    stats: at("wBattleMonAttack"),
+  };
+  const boosts = routine("BadgeStatBoosts");
+  await openReference(page);
+  const result = await page.evaluate(async ({ a, boosts }) => {
+    const { BinjgbEmulator } = await import("/src/emulator/binjgb.ts");
+    const { CrystalAssets } = await import("/src/game/rom/assets.ts");
+    const { Battle } = await import("/src/game/battle/battle.ts");
+    const rom = new Uint8Array(await (document.querySelector("#reference-fixture") as HTMLInputElement).files![0].arrayBuffer());
+    const assets = new CrystalAssets(rom);
+    const reference = new BinjgbEmulator();
+    await reference.load(rom, document.createElement("canvas"));
+    const write = (p: number, v: number) => reference.writeMemory(p, v & 255);
+    const read = (p: number) => reference.readMemory(p);
+    // DI; SP=$c7fe; select the bank; CALL BadgeStatBoosts; sentinel; loop.
+    // The sentinel lives in audio RAM, away from the battle variables.
+    const program = [0xf3, 0x31, 0xfe, 0xc7, 0x3e, boosts.bank, 0xe0, a.romBank & 255, 0xea, 0, 0x20,
+      0xcd, boosts.address & 255, boosts.address >> 8, 0x3e, 0x42, 0xea, 0xf0, 0xc0, 0x18, 0xfe];
+    // Around the cap, a range of Special Attack values for the Glacier
+    // Badge's register bug, and small stats.
+    const statSets = [
+      [1, 2, 3, 4, 5],
+      [10, 20, 30, 40, 50],
+      [887, 888, 889, 900, 998],
+      [999, 999, 999, 999, 999],
+      ...Array.from({ length: 24 }, (_, i) => [100, 150, 200, 180 + i * 30, 300]),
+    ];
+    const failures: unknown[] = [];
+    let compared = 0;
+    for (let badges = 0; badges < 256; badges++)
+      for (const stats of statSets) {
+        const battle = new Battle({
+          data: assets.pokemon, party: [], pokedex: { seen: new Set(), caught: new Set() }, playerId: 1,
+          playerName: "CHRIS", timeOfDay: 1, random: () => 0, text: () => ({}) as never, johtoBadges: badges,
+        });
+        battle.player.mon = { stats: [...stats] } as never;
+        (battle as unknown as { badgeStatBoosts(): void }).badgeStatBoosts();
+        write(a.badges, badges);
+        write(a.linkMode, 0);
+        write(a.tower, 0);
+        stats.forEach((value, i) => { write(a.stats + i * 2, value >> 8); write(a.stats + i * 2 + 1, value); });
+        write(0xc0f0, 0);
+        program.forEach((value, i) => write(0xc000 + i, value));
+        reference.setProgramCounter(0xc000);
+        reference.advanceFrames(1);
+        if (read(0xc0f0) !== 0x42) throw Error("BadgeStatBoosts did not return");
+        const original = stats.map((_, i) => (read(a.stats + i * 2) << 8) | read(a.stats + i * 2 + 1));
+        const native = battle.player.mon.stats;
+        if (original.some((value, i) => value !== native[i])) failures.push({ badges, stats, original, native });
+        compared++;
+      }
+    reference.dispose();
+    return { compared, failures: failures.slice(0, 5), count: failures.length };
+  }, { a: addresses, boosts });
+  expect(result.compared).toBe(256 * 28);
+  expect(result, JSON.stringify(result)).toMatchObject({ count: 0 });
+});