Rates of feeding by the ctenophoreMnemiopsis leidyi were measured in order to evaluate the importance of predation by ctenophores on populations of zooplankton. Copepods, mostlyAcartia tonsa, and ctenophores from the Patuxent River and the harbor at Solomons, Maryland, were used. Rates of feeding were correlated with the experimental containers and the size of the ctenophores. A prediction equation, based on this relationship, was used to estimate the rates of feeding by the population of ctenophores in the Patuxent River. According to the predicted value, ctenophores eat 31% of the population ofA. tonsa daily in the Patuxent River. This value accounts for 52% of the daily summer mortality amongA. tonsa.