SYNOPSIS. It has been 10 years since the taxonomic composition of the important hymenostome genus Tetrahymena has been given overall consideration, and even then the treatment was not extensive. New data of significance have been accumulated and a fresh analysis is clearly in order. Also today we recognize that entire assemblages or combinations of characteristics must be considered in understanding the species‐composition of a protozoan genus, and such an approach has never been uniformly applied in a comparative systematic study of all ciliates belonging to the Tetrahymena group.Three complexes within the genus may now be identified. In the 1st, the pyriformis complex, are placed T. pyriformis (the type‐species), T. setifera, and T. chironomi. In the 2nd, the rostrata group, are assigned T. rostrata, T. limacis, T. corlissi, and T. stegomyiae. In the 3rd, the patula complex, are found T. patula, T. vorax, and T. paravorax. Three additional, formerly independent species are here recognized as doubtful forms: T. faurei, T. glaucomaeformis, and T. parasitica.In spite of some overlapping in certain characters, such as total number of kineties or ciliary meridians, the 3 complexes may be considered distinctive on the basis of constellations of features of taxonomic value, including physiologic and morphogenetic as well as structural characteristics. Yet within each complex it is possible to differentiate clearly a number of separate species. The present analysis in no way closes the door to discovery of still additional, new species of Tetrahymena in the future, but it attempts to lay the groundwork for a uniform usage of combinatiomnas of criteria in comparative taxonomic studies of these and other relatively undifferentiated hymenostome species which possibly will be of some value in the whole area of ciliate systematics.