ABSTRACT: Oligotrich ciliates are a major microzooplankton component in the northern Baltic Sea during summer. The use of wheat starch particles as a tracer-level food source enabled us to simultaneously evaluate in situ clearance rates and food particlesize selection of natural ciliate assemblages. Average clearance rates of 9 ciliates of the genera Lohmanniella, Strobilidium, Strombidium, Tintinnidium and Tintinnopsis on their most favored particle sizes ranged from1.9 to 11.4 ul cell^-1 h^-1. Particle size selectivity was studied by the use of chi²-based selectivity index. Each species showed a specific particle size preference pattern. The optimal particle size varied from 1.4 um(Strombidium sp. 20 um in size) to 9.8 um (Strobilidium sp. 40 um). Most species were able to ingest effectively nanoflagellate-size food, but only 2 of the species showed effective grazing on the smallest particles, suggesting a possibleability to utilize bacteria-size prey. The overall ciliate food size spectrum covered the size range of the most abundant food items in the Baltic summer plankton. Among the ciliates investigated, there appears to be 2 different feeding strategies:specialistic and generalistic, where the ciliates either concentrate on feeding on a narrow size range of food organisms, or use food particles of a wide size range, with little or no preferences within this range.