We calculated diversity indices for planktonic foraminifera in both modern and 18,000 B.P. (late Wisconsin glacial maximum) North Atlantic sediment samples. Modern diversity patterns, especially those based on the Shannon index, reflect present surface circulation. Mapped 18,000 B.P. diversity values reveal a southward migration of the polar front, a shrinking of transitional areas, a southward deflection of the Gulf Stream, and a slight expansion of Sargasso Sea waters. These changes agree with the CLIMAP reconstruction and validate the paleoceanographic utility of diversity maps in epochs prior to the origin of extant species.