The influence of 2,4,5,2',4',5'-hexachlorobiphenyl (HCB) on the growth of Great Lakes phytoplankters Fragilaria crotonensis, Ankistrodesmus falcatus, and Microcystis sp. was investigated in continuous culture under nutrient-sufficient and phosphate (P) limited conditions. Growth rate was inhibited only in F. crotonensis at cellular HCB concentrations as low as 4 to the minus 8th power ng/cell (0.8 micrograms/g dry weight) in nutrient-sufficient cultures; there appeared to be a dose-response relationship between cellular HCB and micrograms. Microcystis sp. and A. falcatus showed no inhibitory effect at cellular HCB levels up to 5 to the minus 8th power ng/cell (3.6 micrograms/g dry weight) and 30 to the minus 8th power ng/cell (11.5 micrograms/g dry weight), respectively. Under P-limited conditions HCB also affected F. crotonensis but not A. falcatus. The initial response of F. crotonensis to HCB addition was a significant increase in micrograms, followed by a sharp decline. In both nutrient-rich and P-limited F. crotonensis the cellular chlorophyll a (chl a) concentration decreased with the accumulation of HCB, yet the 14C assimilation rate per unit chl a increased. Cellular P concentration in the P-limited cells also decreased with the accumulation of HCB. Such responses were not observed in the other species.