AbstractIn P. tetraurelia each cell is determined to express only one of the two complementary mating types,OandE. This determination is under cytoplasmic control and seems to be achieved only by the commitment or noncommitment to the expression of mating typeE. All the previously known mutations affecting the differentiation of mating type prevent the expression of theEmating type (O‐restricted mutations) without affecting thedeterminationprocess. AnE‐restricted mutation was obtained:mtFE. Its phenotypic properties indicate that the mutation affects the determination process itself. When anOcell becomesmtFE/mtFEit acquires theEmating type and anE‐determining cytoplasm. We propose that this constitutive determination for theEmating type is due to the inefficiency of a factor which is normally active in anOcell. This factor would act like a repressor and stabilize theEfunctions under an inactive