The grazing impact of 3 different protozoan species on a mixed bacterial community was studied by means of a simplified and functionally reproducible experimental microbial food web in a 2-stage flow-through system. In the first stage the algae Rhodomonas sp. was grown on an inorganic medium with its accompanying bacterial community (BC) growing on algal exudates. This mixture of algae and bacteria was transferred into 4 second stage vessels: (1) a control, and 3 vessels inoculated with (2) a heterotrophic nanoflagellate, Bodo saltans, (3) a scuticociliate, Cyclidium glaucoma, and (4) a mixotrophic flagellate, Ochromonas sp. Using image analysis techniques we followed the changes in bacterial size distributions and bacterial to protozoan total biovolume ratios over an experimental period of 15 d. In addition, productivity of the grazed and ungrazed BC was measured using [H-3]thymidine and [C-14]leucine. As a consequence of total grazing rates and size-selective feeding we observed 3 different responses of the initially identical BC to grazing of the 3 protists. (1) Low grazing by B. saltans caused a slow decrease of bacterial cell numbers from 14 to 5.9 x 10(6) cells ml(-1), but no significant shift of the mean cell volume (MCV, average 0.107 mu m(3)) and bacterial production. (2) Higher grazing rates of C. glaucoma resulted in the decline of bacterial abundance to 3.3 x 10(6) cells ml(-1) in parallel with a doubling of the MCV to 0.207 mu m(3) and high DNA and protein synthesis rates. Due to the ciliate’s ability to graze also on small prey (