Duration of the interautogamous interval (IAI) was studied in wild and mutant stocks of Paramecium tetraurelia. The investigations revealed slight but statistically significant differences in IAI duration in complementary mating types of some stocks of P. tetraurelia. A certain range of variability characterized the duration of this period in particular stocks. The greatest variability observed in mutant stock d4-108 may result from abnormal, unequal macronuclear divisions frequent in these mutant cells. In none of the investigated stocks is parental age of relatively young, mature for autogamy cells related to the IAI duration in progeny generation.