Threshold concentration, retention efficiency and egg-production in the calanoid copepod Acartia tonsa Dana were examined using the algal species Isochrysis galbana clone T-iso, Dunalietta tertiolecta Butcher, Rhodomonas baltica Karsten, Ditylum brightwellii Grunow and Thalassiosira weissflogii Grun. Feeding and egg-production in A. tonsa was shown to be influenced by the size, quantity and quality of the food particles. The small I. galbana (4.8 μm) were inefficiently retained by A. tonsa and maximum ingestion rates on this species were first obtained at algal concentrations s> 1 μg C · ml−1. However, the highest maximum rate of egg-production was obtained when feeding on this algal species with gross efficiencies of 22 and 38% in terms of carbon and nitrogen, respectively. Egg-production in A. tonsa ceased entirely within 4 days of feeding on a sole diet of D. tertiolecta. D. tertiolecta is similar in size to R. baltica but contained only trace amounts of fatty acids higher than C-18 fatty acids. The diatoms D. brightwellii and T. fluviatilis were retained with maximal efficiency but daily egg-production and gross growth efficiency was lower for these species than for I. galbana and R. baltica.