By identifying the relationships between abundance and metabolic activity in a bacterial community, we advance our knowledge of the marine carbon cycle. In the present study, our goal was to examine the role of protist grazing on the metabolic activity of γ- Proteobacteria in the upwelling system off the Oregon coast. Metabolically active γ- Proteobacteria were identified and quantified by a technique which combines microautoradiography and fluorescence in situ hybridization. The experiment consisted of three treatments: a control, a cubitainer filtered to enhance the effects of grazing, and a cubitainer filtered to reduce the grazing pressure on heterotrophic cells. In the three treatments, changes in the γ-Proteobacteria population were independent of the entire bacterial community. We found different γ-Proteobacteria population patterns in each experimental treatment at the beginning and over the course of a 2 day incubation. In the reduced grazing environment, the increase in γ-Proteobacteria abundance was 11 times greater than the increase in the enhanced grazing environment. This suggests that γ-Proteobacteria are vulnerable to grazing pressures. Additionally, our data suggest that metabolically active γ-Proteobacteria were targeted by grazing protists. Understanding the marine microbial communities through which carbon flows depends upon further defining the relationships between grazers and specific phylogenetic bacterial groups.