A clone of the brackish water ciliate Stentor mĪ ltiformis was found to be colonized simultaneously by three different endocytobionts. A rod like bacterium and a helical bacterium were observed in the cytoplasm enclosed by host membranes, a third bacterium colonized the macronucleus of the host cell. The latter organism occurred in two morphologically different forms. All three endocytobionts were maintained under laboratory conditions.