Feeding and filtration rates of four species of copepods (Pseudocalanus minutus, Temora longicornis, Oithona similis, and Acartia tonsa) from Bras d'Or Lake and Morrison's Pond, Cape Breton Island, N.S., were measured in the laboratory with natural populations of phytoplankton. Adult copepods ingested all sizes of flagellates (1–15 μ in maximum dimension) but 5–15-μ cells were removed most efficiently. Ceratium sp. and Chaetoceros sp., principal spring and winter forms, were not utilized. Feeding rate declined as the number of animals per liter and the duration of the experiment increased. Estimates of feeding rates in nature were derived by extrapolation from the results of laboratory experiments. A comparison of estimates of mean summer primary production at the depths of maximum photosynthesis and daily zooplankton food intake suggested that, at those depths, zooplankton in Bras d'Or Lake consumed all the average daily production whereas 66% was removed in Morrison's Pond. The remaining production in Morrison's Pond probably contributed to the support of large rotifer and ciliate populations not found in Bras d'Or Lake. Natural concentrations of flagellates were more than sufficient to meet copepod respiratory energy requirements under laboratory conditions.