diff --git a/README.md b/README.md
index c861651..060c33b 100644
--- a/README.md
+++ b/README.md
@@ -8,7 +8,7 @@ Build locally with `npm run build` in this directory. Keep the API key on your s
 import { TidelightClient } from "@tidelight/sdk";
 
 const client = new TidelightClient({ apiKey: process.env.TIDELIGHT_API_KEY! });
-const page = await client.listTokenizedEquities({ limit: 20 });
+const page = await client.listTokenizedEquities({ limit: 20, underlyingTicker: "AAPL" });
 for (const instrument of page.data) console.log(instrument.symbol, instrument.underlying_ticker);
 
 for await (const instrument of client.tokenizedEquities({ limit: 50 })) {
diff --git a/src/index.ts b/src/index.ts
index 0be73dc..2042731 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -79,14 +79,17 @@ export class TidelightClient {
     this.transport = options.fetch ?? fetch;
   }
 
-  async listTokenizedEquities(options: { limit?: number; cursor?: string; signal?: AbortSignal } = {}): Promise<InstrumentPage> {
+  async listTokenizedEquities(options: { limit?: number; cursor?: string; underlyingTicker?: string; signal?: AbortSignal } = {}): Promise<InstrumentPage> {
     const limit = options.limit ?? 20;
     if (!Number.isInteger(limit) || limit < 1 || limit > 100) throw new RangeError("limit must be 1 to 100.");
     if (options.cursor !== undefined && !/^[A-Z0-9]{2,32}$/.test(options.cursor)) throw new TypeError("Invalid instrument cursor.");
+    const underlyingTicker = options.underlyingTicker?.toUpperCase();
+    if (underlyingTicker !== undefined && !/^[A-Z][A-Z0-9.]{0,9}$/.test(underlyingTicker)) throw new TypeError("Invalid underlyingTicker.");
     const url = new URL(`${this.baseUrl}/market/instruments`);
     url.searchParams.set("kind", "rtoken");
     url.searchParams.set("limit", String(limit));
     if (options.cursor) url.searchParams.set("cursor", options.cursor);
+    if (underlyingTicker) url.searchParams.set("underlying_ticker", underlyingTicker);
     for (let attempt = 0; ; attempt++) {
       const controller = new AbortController();
       const onAbort = () => controller.abort();
diff --git a/tests/client.test.mjs b/tests/client.test.mjs
index fa7867c..e037d94 100644
--- a/tests/client.test.mjs
+++ b/tests/client.test.mjs
@@ -5,6 +5,17 @@ import { TidelightApiError, TidelightClient } from "../dist/index.js";
 const key = `tl_test_${"a".repeat(43)}`;
 const jobId = "01234567-89ab-4cde-8fab-0123456789ab";
 
+test("filters market instruments by normalized underlying ticker", async () => {
+  let requestUrl = "";
+  const client = new TidelightClient({ apiKey: key, fetch: async url => {
+    requestUrl = String(url);
+    return Response.json({ data: [], next_cursor: null, generated_at: 1, request_id: "market-filter" });
+  } });
+  await client.listTokenizedEquities({ limit: 1, underlyingTicker: "aapl" });
+  assert.match(requestUrl, /underlying_ticker=AAPL/);
+  await assert.rejects(client.listTokenizedEquities({ underlyingTicker: "AAPL?" }), TypeError);
+});
+
 test("starts, polls, and cancels an asynchronous research job", async () => {
   const calls = [];
   const client = new TidelightClient({ apiKey: key, fetch: async (url, init) => {