The green alga Chlamydomonas reinhardtii usually occurs in cultures as single, biflagellated cells. However, C. reinhardtii isknown for its ability to form gelatinous and palmelloid stages that arise as a result of an interaction with its environment.Exponentially growing unicellular C. reinhardtii formed palmelloid colonies rapidly within 25 h when cultured together withtheir enemy the rotifer Brachionus calyciflorus. Consequences of palmelloid formation for population dynamics of both C. reinhardtiiand B. calyciflorus were examined in continuous flow systems. Palmelloids were only formed in a one-stage systemwhere B. calyciflorus grazers and C. reinhardtii prey were cultured together, but not in a two-stage system in which mainly unicellularC. reinhardtii was pumped into a rotifer culture placed in darkness. The rotifer abundance was lower and the algal biomasshigher in the one-stage system compared to the grazing unit of the two-stage system. Inasmuch as palmelloids seemed togive C. reinhardtii cells resistance to grazing, we suggest that at least one of the reasons why C. reinhardtii is capable of formingpalmelloids is to cope with herbivory.