ABSTRACT: Seawater dilution experiments were conducted during spring and fall in the continental shelf region of the northern Gulf of Mexico. Nutrient-enhanced phytoplankton growth rates of 0.7 to 2.2 d^-1 were measured for the entire phytoplanktoncommunity; highest growth rates were associated with &GT8 um cells. Phytoplankton growth was nutrient limited in all May experiments, and &GT8 um phytoplankton, primarily diatoms, showed the strongest response to nutrient addition: their growth ratesincreased the most and reached the highest values. Rates of microzooplankton grazing on the entire phytoplankton community were moderate (0 to 0.7 d^-1). During a given experiment, patterns of grazing on 2 phytoplankton size fractions (&LT8 and&GT8 um) generally differed, and high rates of grazing (&GT1 d^-1) on both &LT8 and &GT8 um cells were sometimes observed. Across all experiments, grazing by microzooplankton averaged 30% of nutrient-enhanced phytoplankton growth. In May, whenphytoplankton growth was strongly nutrient limited, grazing averaged 90% of natural (non-nutrient-enhanced) phytoplankton growth. These data indicate that microzooplankton can be a significant source of phytoplankton mortality, even in eutrophic coastalwaters. The microzooplankton community, excluding cells &LT5 um, comprised primarily heterotrophic dinoflagellates and aloricate choreotrich ciliates. These organisms exhibited high net growth rates (mean = 0.8 d^-1) during experiments at higherirradiance levels. Ingestion of chain diatoms by the dinoflagellate Gyrodinium sp. was observed in preserved samples; such grazing pathways, in which relatively large phytoplankton cells are consumed by protozoa, may be quantitatively important inthis coastal ecosystem. Due to the variety of taxa and feeding mechanisms within the microzooplankton, their grazing impact was not restricted to the smallest phytoplankton cells, indicating that size-based models of trophic structure could yieldmisleading predictions about patterns of energy flow in this coastal ecosystem.