Laboratory studies were designed to define the role of protozoa in the fate of particulate (bacterial) organic carbon. Specific objectives were to measure the effects of selected environmental parameters on protozoan growth rates, to measure organic carbon in bacteria and protozoa, and to quantitate carbon transformation in predator-prey experimental systems. T. Tetrahymena pyriformis altered the amount and form of carbon in the system while growing on bacteria. Of the total organic carbon present at the initiation of the predator-prey experiment (93 mg), 93% was in the bacterial fraction. Within 96 hours, 38% of the carbon was released as CO2; 5% was present as inorganic carbon in the water and the remainder (57%) was present as organic carbon. The organic carbon in the bacterial fraction decreased from 86 to 2 mg within 96 hours, while the carbon in the protozoan biomass increased from 1 to 40 mg. In the bacterial control, 11% of the organic carbon was released as CO2 within 96 hours while negligible amounts of inorganic carbon remained in the water.