SUMMARYTwo‐membered cultures of 3 phytoplankters isolated from Onondaga Lake, grown in defined media under laboratory conditions, were analyzed by electronic particle counts and statistical methods to determine antagonistic, neutral, or stimulatory relationships. Over the range studied (exponential phase of growth, nutrients not limiting), growth ofStaurastrum paradoxumis markedly reduced in the presence ofChlamydomonassp. Growth ofChlamydomonasis not affected by presence ofStaurastrum. The initial concentration ofStaurastrumhas an effect on its subsequent growth. The initial concentration ofChlamydomonasdoes not have an effect on its subsequent growth nor that ofStaurastrum. These effects are not mediated by a filterable factor (unless highly labile) or by competition for a nutrient. They resemble, and perhaps describe, fluctuations of these species in the lak