Natural populations of phytoplankton from Yaquina Bay, Oregon, were incubated in large polyethelene bags in- situ, with and without the natural assemblage of zooplankton. Samples were taken daily for two weeks in April, 1974. The biomass of phytoplankton in the bag without grazers reached values two to three times the biomass of phytoplankton in the bag with grazers. Sixty-eight to 93% of the cells in the bag without zooplankton were Thalassiosira fluviatilis, while the major species in the bag with zooplankton were T. fluviatilis, T. decipiens and Chaetoceros debilis, no one of which ever accounted for more than 40% of the cells. Nitrate-nitrite became depleted two days earlier in the bag without grazers and urea and ammonia values were higher in the bag with the grazers. Primary productivity, per unit cell volume, was higher in the grazed bag following nutrient depletion, suggesting some productivity enhancement by grazers. Physiological parameters indicate that the cells in the bag with zooplankton were richer in carbon, nitrogen and chlorophyll a, a higher percentage of the chlorophyll was degraded to phaeophytin and carbon to nitrogen ratios were lower. Data obtained from the bag with zooplankton were similar in range to data from the bay. The results support the contention that zooplankton can have a large effect on phytoplankton in terms of biomass, species composition, productivity and physiological state. / Graduation date: 1975