Biochemical substances released from Daphnia galeata induced colony formation in the green alga Scenedesmus acutus. Normally this strain consisted mainly of single cells in cultures. However, if exposed for 48 h to either water with live Daphnia or to 0.1‐µm filtered water from a culture with Daphnia present, these unicellular “Chodatella” stages were induced to form colonies (coenobia). Colony induction was not unique to Daphnia; other zooplankters (rotifers and copepods) were able to induce colonies in S. acutus as well. This morphological response could also be evoked when Scenedesmus was exposed to 0.1‐µm filtered lake water during high zooplankton abundances. Especially during early spring, a clear relationship was found between rotifer abundance and colony formation in our test alga in the laboratory. Filtered lake water, incubated nonaxenically for at least 2 d at 20°C, did not induce colony formation, possibly due to microbial degradation. The morphological changes in Scenedesmus could promote grazing resistance in small zooplankters and can be interpreted as an adaptive antipredator strategy.