Species distributions may be determined by a variety of biotic and abiotic factors. The model ciliate Tetrahymena thermophila exhibits an unusual distribution limited to the eastern United States. In this study, we assess the roles of species interactions and habitat requirements in driving this restricted distribution by studying laboratory populations of T. thermophila and Tetrahymena gruchyi. We find that priority effects and habitat requirements are likely to be important factors driving the distribution of Tetrahymena species.