We compared the characteristics of ingestion of Prochlorococcus and Synechococcus by the marine heterotrophic nanoflagellate Pseudobodo sp. and by a mixed nanoflagellate culture (around 3 microm in size) obtained from an open sea oligotrophic area. Maximum ingestion rate on Synechococcus (2.7 Syn flagellate(-1) h(-1)) was reached at concentrations of 5 x 10(5) Syn mL(-1) and decreased between 6 x 10(5) and 1.5 x 10(6) Syn mL(-1). In order to validate laboratory data, one set of data on Synechococcus grazing was obtained during a field study in the oligotrophic northeastern Mediterranean Sea. Ingestion rates by heterotrophic nanoflagellates were related to Synechococcus abundance in the water, and the feeding rate showed a clear diel rhythm with consumption being highest during the night, declining during the day hours, and being lowest at dusk. Ingestion rates on Prochlorococcus increased linearly for the whole range of prey density used (i.e., from 1 x 10(3) to 3 x 10(6) Proc mL(-1)), with maximum ingestion of 6.7 Proc flagellate(-1) h(-1). However, for prey concentrations in the range of 10(3)-10(5), which are usually encountered in aquatic systems, ingestion rates were significantly less than on Synechococcus. In our experiments, both Prochlorococcus and Synechococcus proved to be poor food items for support of nanoflagellate growth.