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 the changes in the phytoplankton flora with time. Discrete water samples from 28 stations were collected for identification and enumeration of phytoplankton. The spring increase in WCR 828 occurred from late April to mid-May and was multiphasic; early periods were dominated by the diatoms Minidiscus trioculatus (4–5 μm diam.) and a small Thalassiosira, possibly T. bulbosa, while later periods were dominated by a small (2–3 μm) biflagellate. In June, another diatom concentration was detected at ring center, but this one was dominated by Chaetoceros cf. vixvisibilis and Leptocylindrus danicus. After interactions with and overwashes by the Gulf Stream and Slope Water in July, diatom numbers in the surface waters of the ring in August were greatly reduced relative to June, and no single species dominated. Changes in phytoplankton abundance in the ring core occurred on different time sequences from changes in the surrounding Slope Water or in the source water, the Sargasso Sea. The dominant taxa in the ring changed rapidly, on time scales of 1.5 months or less (intercruise time period). Successional changes were more important in altering the phytoplankton composition during the first two cruises, while sequential changes characterized the end of the study period. The ring center showed dramatic differences from its source water just 2 months after ring formation but remained distinct from the Slope Water for 4–5 months.