During the spring and summer of 1982, Gulf Stream warm core ring (WCR) 82B was sampled during four cruises from April to August to investigate phytoplankton distributional patterns. Discrete water samples from 28 stations were collected for identification and enumeration of phytoplankton. In April, when the water column was well mixed to 350 m, quantitative samples clustered by station when the 100 most frequently observed taxa were used as variables, indicating fairly unique assemblages at each station that were consistent with depth. Two transects across the ring in June showed a symmetrical diatom abundance maximum, dominated by Chaetoceros cf. vixvisibilis (maximum abundance 31,900 cells l–1) and Leptocylindrus danicus (maximum abundance 21,000 cells l–1), situated in the surface water at ring center. Dinoflagellate and coccolithophorid maxima were situated slightly deeper than the diatom maximum, in the seasonal thermocline from 20 to 35 m. A biomass maximum observed in a Shelf Water entrainment feature wrapping around the eastern perimeter of the ring contained elevated numbers of coccolithophorids and coccoid, unicellular monads (1–3 μm in diameter) and was thus compositionally distinct from the ring center biomass maximum. In July and August the ring underwent numerous interactions with and overwashes by the Slope Water and Gulf Stream. August samples from the ring, Sargasso Sea, Gulf Stream, and Slope Water all contained similar taxa and abundances. Different phytoplankton groups may be responding to different nutrient input mechanisms at the ring edge and center. Diatom maxima at ring center may form as a result of pulsed nutrient input from storms and a slight upwelling due to the gradual relaxation of the thermocline as the ring ages, while concentrations of ultraplanktonic algae (monads, coccolithophorids) toward the ring margin may result from near steady-state nutrient input along sloping isopycnals and/or advection from the ring exterior.