The rates of ingestion of bacteria and of accumulation of bacterial biomass by hungry Pteridomonas danica andParaphysomonas imperforata were measured using dual radioactive-labelled bacteria in experiments lasting 4--8 h. Pteridomonas continuously consumed 4--5 bacteria h - 1 throughout experiments lasting 8 h, irrespective of bacterial concentration above a threshold of about 5 x 10 5 bacteria ml - 1 , and continued to catch bacteria even below this density. The clearance rate of about 1 nl cell - 1 h - 1 at higher bacterial concentrations increased three or four times as bacterial numbers fell. Paraphysomonas cells, with only half the biomass of Pteridomonas, ingested up to 10 bacteria h - 1 at high bacterial concentrations, and gradually reduced the feeding rate, effectively ceasing to feed at 10 6 bacteria ml - 1 ; their initial clearance rate of 1--2.5 nl cell - 1 h - 1 subsequently fell as low as 0.1 nl cell - 1 h - 1 . Estimation of feeding rate by extrapolation from short-term experiments on such flagellates requires extreme caution. These flagellates, starved to levels typical of the natural environment, accumulated ingested bacterial biomass at an efficiency of between 16 and 21%, indicating that in nature they would recycle 80% or more of the nutrients contained in their food.