Grazing experiments with freshly caught plankton from Narragansett Bay were performed at weekly or biweekly intervals through most of a seasonal cycle. The laboratory rates (obtained under simulated natural conditions) were multiplied by the zooplankton concen- tration in the natural environment and the effects of grazing on the phytoplankton commu- nity were assessed, Grazing was most extensive in spring and early summer; phytoplankton production was believed to be high and a large standing crop of Skeletonema costatum persisted. The grazing population was severely reduced during summer, but the phyto- plankton standing crop did not increase markedly and some other factor must have been limiting primary production at this time. During fall, decreasing light allowed little phyto- plankton growth and mild grazing kept the standing crop at a minimum, This grazing ceased in December and with the release in grazing pressure the winter-spring diatom flowering began.