Analysis of late Paleocene to early Eocene planktic foraminifera from DSDP Site 577 indicates the presence of two and possibly three isotopically and morphologically distinct assemblages. The surface dwelling assemblage includes species with pustulate-muricate wall texture (e.&, species of the genera MorozoveUa, Acarinina, lgorim, and Murikoglobigerina). This assemblage is characterized by light W80 values and large interspecific S’3C variation. It accounts for 82% of the population. The subsurface dwelling assemblage includes species with cancellate-pitted and smoothed-granulate wall textures (e+, species of the genera Subbotina, LLTurborotalia,” Planorotalites, Pseudohastigerina, and Chiloguembelina). This assemblage is characterized by heavier 6‘80 values and small interspecific SI3C variation. It accounts for 14% of the population. The questionable deep dwelling assemblage includes some species with pitted wall texture (e.g., species of the genus “Planorotalites”). This assemblage is characterized by the heaviest SI8O values and accounts for only 4% of the population.