Members of the ciliate group of protozoans are often observed in mass cultures of rotifers. In particular, Euplotes and Vorticella are common contaminating species. In this study, I examined the effect of the ciliates Euplotes sp. and Vorticella sp. on the growth of the rotifer Brachionus rotundiformis by conducting inter-specific and tripartite-specific mixed-culture experiments. The growth of rotifers was suppressed in co-existence with Euplotes sp. compared with monocultures of rotifers. However, Vorticella sp. promoted rotifer growth. Moreover, Vorticella sp. improved the growth of rotifers suppressed by Euplotes sp. contaminants. In 5-L semi-mass cultures of rotifers, growth of the contaminating protozoan Euplotes sp. was heavily suppressed by Vorticella sp. The stable maintenance of the rotifer culture ecosystem can be achieved by manipulating the types of contaminating protozoan species.