We examined effects of the invasive, toxigenic haptophyte Prymnesium parvum on grazing rates, feeding behaviors, and life-history characteristics of clonal lineages of three daphniid zooplankton species. Grazing experiments revealed similar clearance rates for P. parvum and a common green alga. Behavioral observations revealed no significant effects of P. parvum on daphniid feeding behaviors after 30 min, but major declines in appendage beat rates after 1 h. Chronic exposure (10 d) to P. parvum resulted in severe reductions in daphniid growth rates, age at first reproduction, fecundity, and survivorship at densities as low as 7750 cells mL-1. Thus, in addition to direct fish mortality during P. parvum blooms of 50,000-200,000 cells mL-1, the entire food web of an invaded system may be subjected to potentially severe negative consequences even at nonbloom densities of P. parvum. ?? 2011, by the American Society of Limnology and Oceanography, Inc.