Two new mutant lines of Tetrahymena thermophila (T. pyriformis, syngen 1), each conferring resistance to a different agent, are described. Resistance to cycloheximide and 6-methylpurine are each determined by dominant genes, ChxA2 and Mpr; the traits show phenotypic assortment. The method used to select these mutations, the critical importance of backcrossing to wild type following mutagenesis, and the utility of these marker genes in further mutagenic selection schemes and studies of the sexual cycle of Tetrahymena are noted.