The quantitative uptake of latex beads of different sizes and of live cells by 14 species of ciliates was studied. The functional response (uptake rate as function of food particle concentration) can be fitted to a hyperbolic function and this can be explained in terms of the function of the mouth apparatus. Each species shows a distinct size spectrum of particles which are retained and ingested. These size spectra may be explained by mouth morphology, and particle size selection may play a role for niche separation of coexisting ciliates. Most bacterivorous holotrich ciliates retain particles down to 0.2μm and in one case down to 0.1μm; they retain particles between 0.3 and 1μm most efficiently. The spirotrich ciliates investigated do not retain particles smaller than 1-2μm.
