Abstract: This paper reported on the periodicity of microfilariae in the blood of 53 cases with bancroftian filariasis and 21 cases with malayan filariasis in various parts of Sichuan Province. The periodicity indices of the mf. of W. bancrofti and B. malayi were 103.85% and 112.08%, respectively. The best estimate of the peak hour was 1:06 a.m. in patients with bancroftian filariasis and 1:18 a.m. in patients with malayan filariasis. It was demonstrated that the mf. of both W. bancrofti and B. malayi in Sichuan exhibited typical nocturnal periodicity.