We examined the combined effects of grazer infochemicals and nutrient status on colony development of Phaeocystis globosa cultures grown under nitrogen and phosphorus (NP)-sufficient, P-deficient, and N-deficient conditions exposed to high and low Acartia spp. density filtrates. Changes in colony development relative to controls receiving no grazer signals were estimated. P. globosa colony development responded to grazer infochemicals regardless of nutrient status, although the expression of the response varied between nutrients. Significant colony suppression (in terms of percent of cells allocated to colonies) occurred in both NP-sufficient and P-deficient experiments, with the response being dependent on the density of grazers for NP-sufficient cells. The percent of cells in colonial form in N-deficient P. globosa decreased in response to low grazer density filtrates but increased in response to high grazer density filtrates. These opposite results for the N-deficient experiment are related to a high mortality of Acartia in the high grazer density filtrate treatment, which may affect the infochemicals released from such grazers.