Infective larvae of nocturnally subperiodic Wuchereria bancrofti were obtained from laboratory-raised Aedes togoi mosquitoes which had fed on an infected human volunteer from Kanchanaburi, Thailand, and 471 of them were inoculated subcutaneously into the inguinal regions of an immunosuppressed Presbytis melalophos. Microfilaremia was first detected 287 days after inoculation, and 77 days later, at 364 days of infection, the number of microfilariae determined at 2-hour intervals reached a maximum of 619/20 mm3 at 4 hours past midnight, and the daytime levels were 11-35/20 mm3; the corresponding levels in the human volunteer were 121 and 13-32/20 mm3. Thus the nocturnal periodicity was relatively greater in the monkey than it was in the human host. Eight days after the periodicity study the monkey died and 77 adult worms were recovered from the popliteal (4), inguinal (58), iliac (6) and para-aortic (9) lymph nodes and 1 each from the ovary and liver. Of 56 females, only half were gravid.