ABSTRACT: The dinoflagellate Cochlodinium polykrikoides Margalef has formed dense blooms and caused severe fish kills on a global scale in recent decades. Survivorship, feeding, and fecundity of the copepod Acartia tonsa Dana fed C. polykrikoides were investigated to assess potential impacts of these harmful events on herbivores. Survivorship of female A. tonsa was significantly reduced with increasing C. polykrikoides concentrations from 900 to 4700 µg C l^–1 (500 to 2600 cells ml^–1). Copepods suffered 100% mortality within 1.5 d at C. polykrikoides concentrations of 3300 and 4700 µg C l^–1 (1800 and 2600 cells ml^–1), which are within the range of bloom densities of this alga. Stage-specific mortality of A. tonsa showed that copepod susceptibility to C. polykrikoides decreased with development. Two bioassay experiments suggested that copepod mortality was due to multiple harmful compounds produced by C. polykrikoides. Ingestion rates of A. tonsa fed C. polykrikoides were 25 to 60% lower than ingestion rates of non-toxic Rhodomonas lens Pascher and Ruttner when the food concentrations ranged from 150 to 1500 µg C l^–1. C. polykrikoides supported higher egg production rates of A. tonsa than R. lens at the low algal concentrations (18 to 180 µg C l^–1), while egg production rates of A. tonsa fed C. polykrikoides were significantly less than those fed R. lens when the concentrations increased from 360 to 1080 µg C l^–1. Egg hatching success of A. tonsa fed C. polykrikoides ranging from 90 to 1080 µg C l^–1 was very low (20 to 43%) compared to the higher values when fed R. lens (83 to 100%). Egg sizes of A. tonsa fed C. polykrikoides were significantly lower than those fed R. lens. All of these deleterious consequences may lead to A. tonsa population collapses during C. polykrikoides blooms.