diff --git a/.changeset/uplc-bool-js-boolean.md b/.changeset/uplc-bool-js-boolean.md
new file mode 100644
index 00000000..0f828324
--- /dev/null
+++ b/.changeset/uplc-bool-js-boolean.md
@@ -0,0 +1,5 @@
+---
+"@evolution-sdk/evolution": patch
+---
+
+Fix the UPLC flat encoder encoding a Bool constant given as a plain JS boolean `false` as true. `ConstantValue` permits a JS boolean, but the Bool encoder read `.index` off the value, which a boolean does not have, so `con bool false` and `con bool true` produced identical flat bytes. The encoder now handles the boolean form directly; the decoded `{ index }` form encodes exactly as before.
diff --git a/packages/evolution/src/UPLC.ts b/packages/evolution/src/UPLC.ts
index 760f9ef8..ef8d89d3 100644
--- a/packages/evolution/src/UPLC.ts
+++ b/packages/evolution/src/UPLC.ts
@@ -950,8 +950,11 @@ const encodeData = (state: EncodeState, dataType: DataType, value: ConstantValue
   } else if (dataType === "Unit") {
     // No encoding needed
   } else if (dataType === "Bool") {
-    const boolVal = value as { index: bigint }
-    pushBit(state, boolVal.index === 0n ? 0 : 1)
+    // ConstantValue permits a plain JS boolean as well as the decoded
+    // { index } form; a boolean has no .index, so reading it would
+    // encode false as true.
+    const bit = typeof value === "boolean" ? (value ? 1 : 0) : (value as { index: bigint }).index === 0n ? 0 : 1
+    pushBit(state, bit)
   } else if (dataType === "Data") {
     encodeByteString(state, value as Uint8Array)
   } else if ("list" in dataType) {
diff --git a/packages/evolution/test/UPLC.test.ts b/packages/evolution/test/UPLC.test.ts
index 349ea76f..720473cb 100644
--- a/packages/evolution/test/UPLC.test.ts
+++ b/packages/evolution/test/UPLC.test.ts
@@ -167,6 +167,51 @@ describe("UPLC Module", () => {
       const decoded2 = UPLC.fromFlatBytes(reencoded)
       expect(decoded2.version).toBe(program.version)
     })
+
+    describe("Bool constants given as plain JS booleans", () => {
+      const boolProgram = (value: UPLC.ConstantValue) =>
+        new UPLC.Program({ version: "1.0.0", body: UPLC.constantTerm("Bool", value) })
+
+      const decodedConstant = (bytes: Uint8Array): UPLC.ConstantValue => {
+        const decoded = UPLC.fromFlatBytes(bytes)
+        if (decoded.body.type !== "Constant") throw new Error("expected a Constant term")
+        return decoded.body.value
+      }
+
+      it("encodes JS false as False, not True", () => {
+        const falseBytes = UPLC.toFlatBytes(boolProgram(false))
+        const trueBytes = UPLC.toFlatBytes(boolProgram(true))
+        expect(Bytes.toHex(falseBytes)).not.toBe(Bytes.toHex(trueBytes))
+        expect(decodedConstant(falseBytes)).toEqual({ index: 0n, fields: [] })
+      })
+
+      it("encodes JS true as True", () => {
+        const trueBytes = UPLC.toFlatBytes(boolProgram(true))
+        expect(decodedConstant(trueBytes)).toEqual({ index: 1n, fields: [] })
+      })
+
+      it("matches the { index } representation byte for byte", () => {
+        expect(Bytes.toHex(UPLC.toFlatBytes(boolProgram(false)))).toBe(
+          Bytes.toHex(UPLC.toFlatBytes(boolProgram({ index: 0n, fields: [] })))
+        )
+        expect(Bytes.toHex(UPLC.toFlatBytes(boolProgram(true)))).toBe(
+          Bytes.toHex(UPLC.toFlatBytes(boolProgram({ index: 1n, fields: [] })))
+        )
+      })
+
+      it("roundtrips JS booleans nested in a list of Bool", () => {
+        const program = new UPLC.Program({
+          version: "1.0.0",
+          body: UPLC.constantTerm({ list: "Bool" }, { items: [false, true] })
+        })
+        expect(decodedConstant(UPLC.toFlatBytes(program))).toEqual({
+          items: [
+            { index: 0n, fields: [] },
+            { index: 1n, fields: [] }
+          ]
+        })
+      })
+    })
   })
 
   describe("Schema Transform - FromFlatBytes", () => {