diff --git a/src/us_street/Candidate.ts b/src/us_street/Candidate.ts
index 5200518..1f3efff 100644
--- a/src/us_street/Candidate.ts
+++ b/src/us_street/Candidate.ts
@@ -178,6 +178,7 @@ export interface RawUsStreetCandidate {
 	last_line?: string;
 	delivery_point_barcode?: string;
 	smarty_key?: string;
+	smarty_key_ext?: string;
 	components?: RawUsStreetComponents;
 	metadata?: RawUsStreetMetadata;
 	analysis?: RawUsStreetAnalysis;
@@ -193,6 +194,7 @@ export default class Candidate {
 	lastLine: string;
 	deliveryPointBarcode: string;
 	smartyKey: string;
+	smartyKeyExt: string;
 	components: UsStreetComponents;
 	metadata: UsStreetMetadata;
 	analysis: UsStreetAnalysis;
@@ -207,6 +209,7 @@ export default class Candidate {
 		this.lastLine = responseData.last_line ?? "";
 		this.deliveryPointBarcode = responseData.delivery_point_barcode ?? "";
 		this.smartyKey = responseData.smarty_key ?? "";
+		this.smartyKeyExt = responseData.smarty_key_ext ?? "";
 
 		this.components = {} as UsStreetComponents;
 		if (responseData.components !== undefined) {
diff --git a/tests/us_street/test_Candidate.ts b/tests/us_street/test_Candidate.ts
index b988283..401a561 100644
--- a/tests/us_street/test_Candidate.ts
+++ b/tests/us_street/test_Candidate.ts
@@ -12,6 +12,7 @@ describe("A match candidate", function () {
 			last_line: "5",
 			delivery_point_barcode: "6",
 			smarty_key: "0000",
+			smarty_key_ext: "0",
 			components: {
 				urbanization: "7",
 				primary_number: "8",
@@ -96,6 +97,7 @@ describe("A match candidate", function () {
 		expect(candidate.lastLine).to.equal("5");
 		expect(candidate.deliveryPointBarcode).to.equal("6");
 		expect(candidate.smartyKey).to.equal("0000");
+		expect(candidate.smartyKeyExt).to.equal("0");
 
 		expect(candidate.components.urbanization).to.equal("7");
 		expect(candidate.components.primaryNumber).to.equal("8");
diff --git a/tests/us_street/test_Client.ts b/tests/us_street/test_Client.ts
index 52f29e3..f8f3bcb 100644
--- a/tests/us_street/test_Client.ts
+++ b/tests/us_street/test_Client.ts
@@ -257,6 +257,17 @@ describe("A US Street client", function () {
 		});
 	});
 
+	it("maps smarty_key_ext from response to smartyKeyExt on candidate.", function () {
+		const expectedMockPayload = [{ smarty_key_ext: "0", input_index: 0 }];
+		let mockSender = new MockSenderWithResponse(expectedMockPayload);
+		const client = new Client(mockSender);
+		let lookup = new Lookup();
+
+		return client.send(lookup).then((_response) => {
+			expect(lookup.result[0].smartyKeyExt).to.equal("0");
+		});
+	});
+
 	it("attaches match candidates to their corresponding lookups.", function () {
 		const expectedMockPayload = [
 			{ delivery_line_1: "Address 0", input_index: 0 },