The circadian photoaccumulation rhythm of thirty strains of Paramecium bursaria collected at different places in Japan and China were measured with a microcomputer assisted data collection apparatus. Although most strains showed a period of 23–26 hours in LL, we found two strains of conspicuously different periods; a short period strain (UK1, 21.8 hr) and a long period strain (T316, 28.7 hr). F1 progeny from a cross between the short and the long period strains showed an intermediate period of about 24.7 hours (range 22.5–25.8 hr). The character was not distributed in a Mendelian ratio among the F1 progeny. We isolated a mutant (E2) with short period (21.8 hr) from the stock strain Kz1 by treatment with nitrosoguanidine (MNNG). The progeny of crosses between E2 and UK1, and between E2 and T316 exhibited the short period and the normal period phenotype respectively. Moreover, the progeny from a cross between E2 and a wild type strain (Sj2w) became sexually mature about 25 fissions after conjugation. This length of immaturity is much shorter than that of the progeny from wild type strains (about 50 fissions). This early maturation character was inherited to progeny in a Mendelian ratio. Homozygotes for the early maturation allele (EM2) exhibited mating ability about 15 fissions after conjugation. These data suggest that there is a correlation between the period length of the circadian rhythm and the length of immaturity after conjugation in Paramecium bursaria.