Abstract Rates of predation by adult female and male Corycaeus amazoniens F. Dahl feeding upon specific naupliar stages (N1–N6) of Acartia tonsa Dana were significantly influenced by prey concentration, prey stage ( = size), and sex of predator. Predation rates were linear functions of prey concentration, but mean predation rate by females was approximately double that for males. Some stages of nauplii were consumed at higher rates than others, and ability to capture and ingest naupliar stages, particularly older ones (N4–N6), differed with sex of predator. Rates of consumption of nauplii in the presence and absence of alternative phytoplankton food were not significantly different, suggesting that Corycaeus amazonicus is primarily carnivorous. Rates of consumption of nauplii from the present study, combined with data on the abundances of C. amazonicus and copepod nauplii in continental shelf waters, suggest that predation by C. amazonicus removes < 1% of natural populations of nauplii per day.