Optimum concentrations of food organisms were selected from results of C-labeled phytoplankton feeding experiments with Acartia clausi adults (Zilloux, 1968). These copepod fed upon Rhodomonas very efficiently and approached maximum ingestion rate when the food concentration reached 50,000 cells ml. Ingestion of Isochrysis occurred at a relatively low rate,until clumping occurred at concentrations well in excess of 50,000 cells/ml. This smaller alga is probably of greater importance to developmental stage of AcartiaB, borne on setae of adult feeding appendages, are separated considerably more than are the comparable bristles of nauplii.