Phytoplankton species composition and cell concentrations were determined from samples taken during an ongoing bloom of the toxic dinoflagellate Gymnodinium breve that occurred in November 1987 in North Carolina Core Sound and coastal waters. Despite G. breve concentrations that reached 3.27 x 10^5 cells L-1, this was not a monospecific bloom. Total phytoplankton abundance increased regardless of G. breve concentrations. Principle components analysis and multiple regression analyses identified taxa showing positive and negative associations with G. breve. Phytoplankton with increased concentrations during this bloom included the diatoms Actinoptychus senarius, Odontella aurita, Rhizosolenia stolterfothii, and the dinoflagellates Ceratium furca, C. teres, Katodinium rotundatum, Prorocentrum minimum, Protoperidinium divergens, and Scrippsiella trochoidea. Decreasing in abundance during this bloom were several Cyclotella spp., Triceratium favus, Gymnodinium sp., and Oscillatoria erythraea.