AbstractThemtFEmutation isolated in Paramecium tetraurelia affects mating type differentiation, trichocyst excretion, and viability. Its effect on mating type has already been shown to correspond to a restriction to theEmating type interpreted by an inefficiency of nuclearO‐determining factors. In this paper we study the other two phenotypic characteristics whose hereditary transmission displays two unusual features. (1) In crosses between a wild‐type strain and the mutant strain, the mutant characteristics do not reappear in F2 in the wild‐type cytoplasmic lineage but only in F3 after the homozygous clones have undergone an additional nuclear reorganization. (2) Some F2 wild‐type clones, in the mutant cytoplasmic lineage, retain some of the phenotypic characteristics of the mutant. We propose that themtFgene product plays a role in the control of several macronuclearly differentiated fu