Monthly samples of stratified plankton tows taken from the slope waters off Cape Cod nearly 25 years ago are used to describe the seasonal succession of planktonic foraminifera and their oxygen isotope ratios. The 15°C seasonal cycle of sea surface temperature (SST) accounts for a diverse mixture of tropical to subpolar species. Summer samples include various Globigerinoides and Neogloboquadrina dutertrei, whereas winter and early spring species include Globigerina bulloides and Neogloboquadrina pachyderma (dextral). Globorotalia inflata lives all year but at varying water depths. Compared with the fauna in 1960–1961 (described by R. Cifelli), our samples seem warmer. Because sea surface salinity varies little during the year, δ¹⁸O is mostly a function of SST. Throughout the year, there are always species present with δ¹⁸O close to the calculated isotopic equilibrium of carbonate with surface seawater. This raises the possibility that seasonality can be estimated directly from the range of δ¹⁸O in a sediment sample provided that the δ¹⁸O-salinity relationship is the same as today.