The hypothesis that lower retention efficiencies of filterfeeding copepods for small particles should result in different ingestion rate versus food concentration curves for different-sized foods was tested using Ternora longicornis (M/iller) fed natural phytoplankton. The copepods were fed different natural phytoplankton assemblages, which varied in their species and size distribution. Volume ingestion rates were an asymptotic function of food concentration, with maximum ingestion rates measured at food concentrations exceeding 5 to 10 x 106 lJm a ml -I , which were less than those occurring in the natural waters in which the copepods and phytoplankton were collected. Maximum volume ingestion rates increased linearly by a factor of 3.5, as the diameter of the particle forming the peak in the food size distribution increased from 5 !am (primarily microflagellates) to 30 l~m (mostly large diatoms). These results suggest that natural and pollutant-induced size reductions in natural phytoplankton could markedly decrease the volume of food consumed by Nter-feeding copepods.