The staining properties of the vital dyes Janus green, brilliant cresyl blue, Nile blue, toluidine blue, Bismarck brown, methyl green, and vital red, isamine blue, trypan red, carminate of Na, neutral fuchsin, rhodamine, neutral red, and methylene blue were tested on Opalina sp.? This organism reduces Janus green, brilliant cresyl blue, Nile blue, toluidine blue, and methylene blue in its protoplasm when placed in H2 from which all O2 is excluded. Paramecium caudaium shows similar properties as regards methylene blue. Since parasitic Opalina and free-living Paramecium both possess reducase it is indicated that its possession is not an adaptation to the parasitic mode of life.