We have compiled published planktonic foraminiferal oxygen isotope values from the tropical to subtropical Atlantic and Pacific for the late Holocene and Last Glacial Maximum (LGM). We interpret the spatial distribution of Δ18O values in terms of meridional density gradients in the LGM surface ocean and discuss implications on tropical ocean thermocline structure. In both oceans, glacial Δ18O values display a pattern of interhemispheric asymmetry evidenced by increasingly higher glacial Δ18O values toward northern latitudes. The Δ18O-derived density estimates suggest that the northern subtropical surface ocean density was higher during the LGM than today, which has significant implications on LGM thermocline ventilation. In the Atlantic, steeper density gradients in the northern subtropics during the LGM are consistent with a more compressed subtropical gyre and thermocline water formation farther south than today. In the Pacific the steeper density gradients suggest a significant reduction in freshwater fluxes to the northern tropical and subtropical regions, which raises the possibility of a more significant component of Northern Hemisphere source waters to the tropical Pacific thermocline. These predictions could be tested with numerical ocean models that assess the structure of the tropical thermocline in the glacial ocean. ¿ 2000 American Geophysical Union