We estimated In situ grazing rates of heterotrophic nanoplankton (HNAN) on bactenoplankton in a salt marsh estuary using a select~ve metabolic inhibitor technique. Seasonal experiments with prokaryohc inhibitors showed a significant, positive relation between bacterial abundance and HNAN grazing rates. On average, only 40 to 4 5 '/o of bacterioplankton produchon in the estuary appeared to be consumed by phagogrophic Protozoa <20 pm in size. Eukaryotic inhibitor experiments suggested that protozoan grazlng can have a positive feedback effect on bacterioplankton production, and that nitrogen regeneration by Protozoa may be of particular importance to bacterial growth. Although eukaryotic inhibitors did not prove to be rouhnely useful to estimate protozoan grazing rates on bacterioplankton, they may contribute to understanding qualitahve control mechanisms in the microbial food web.