Frequency distribution of Holocene planktonic foraminifera was determined for the Gulf of Mexico and the North Atlantic Ocean by counts of 246 trigger core tops. These were compared with planktonic foraminifera that were counted from the 18,000-yr isochron in 12 piston cores from the western Gulf of Mexico. Factor analysis was used to define five groups of faunal frequency data and to predict summer and winter temperatures and salinities. Circulation of near-surface water in the Gulf of Mexico 18,000 yr ago, during the last glacial maximum, was significantly different from that of the present. Surface waters supported a cooler subtropical fauna during the glacial maximum, compared with the warm subtropical fauna of the present. Surface temperature was 1° to 2°C cooler and salinity was 0.4 to 0.5 per mil higher than now. In the present gulf, winter isotherms trend northeasterly, and summer surface waters are nearly homogeneous. In the glacial western gulf, isotherms trended northerly during winter and summer. In the present gulf, isohalines parallel the basin edge, and the gradient decreases coastward. In the glacial gulf, although isohalines also followed the basin margin, the gradient increased coastward. These patterns of isotherms and isohalines suggest that an anticyclonic current system existed in the western Gulf of Mexico during the last glaciation.