Three strains of the nonspiny Scenedesmus acutus Meyen, one strain of the nonspiny S. falcatus Chodat, and three strains of the spined Scenedesmus quadricauda (Turpin) Brébisson were cultured in standard medium and in medium containing filtered water from a culture of the grazer Daphnia. Nonspiny strains were examined to detect grazer-induced colony formation. Spine length was measured in spined strains. Strains varied considerably in their response to chemicals from Daphnia. Three nonspiny strains showed rapid formation of four- and eight-celled coenobia in the presence of Daphnia water, whereas all nonspiny cultures remained unicellular in standard medium. By contrast, no grazer-induced colony formation occurred in spined strains. One spined strain formed eight-celled coenobia both in the absence and the presence of Daphnia medium. No evidence for grazer-induced spine formation was found. Growth rates were not affected by Daphnia water in all strains, but differed among strains. The phenomenon of grazer-induced coenobium formation seems widespread, but not universal within Scenedesmus.