Dipetalonema viteae in Meriones unguiculatus usually completed the 3rd molt at 7 days and the 4th molt at 23 days of infection. Insemination, first recorded at 23 days of infection, occurred most frequently at 25 to 28 days. Microfilariae were first seen in the uterus at 42 days and in the peripheral blood at 47 days. The minimal time between insemination and detection of microfilariae in the blood was 22 days, and between implantation of mature uninseminated females together with mature males and the detection of microfilaremia, 26 days. In inseminated females isolated soon after the first mating, sperm was depleted as early as 14 days and as late at 85 days, indicating that for continuous fertility when few worms are present frequent mating is required. Mated and unmated worms of both sexes lived for periods cf up to about 2 years. Studies on the development of filariae in the vertebrate host have emphasized mainly the gross morphological changes and molting of the worms, and the duration of prepatency and patency of the infection. More detailed observations on the development of the organ systems were reported for Dirofilaria immitis by Orihel (1961) and for Brugia pahangi by Schacher (1962). Development of the microfilaria of different species was observed by Kotcher (1941) and McFadzean and Smiles (1956), and spermiogenesis was described by Taylor (1960). Observations on the filarial reproductive system have included studies by Webber (1954a, b), in vitro examinations by Weinstein and Sawyer (1961) and by Raghavan et al. (1956). Hawking (1954) studied the reproductive capabilities of Litomosoides carinii both in vitro and in vivo. Received for publication 18 September 1973. * Conducted under the sponsorship of the Commission on Parasitic Diseases, Armed Forces Epidemiological Board, and supported by the Office of the Surgeon General, Department of the Army, and by NIH research grant AI-04919 from the NIAID. Address reprint requests to: Dr. Paul C. Beaver, Department of Tropical Medicine and Parasitology, Tulane Medical Center, 1430 Tulane Avenue, New Orleans, La. 70112. t Present address: Department of Biology, University of Southern Mississippi, Southern Station, Box 18, Hattiesburg, Mississippi 39401. Dipetalonema viteae (Krepkogorskaja, 1933), a natural parasite of some members of the Gerbillidae (Meriones spp. and Rhombomys opimus), was studied by Chabaud (1954), Worms et al. (1961), and Bain (1967). Weiss (1970) compared the rate of development and persistence of infection in various laboratory hosts, mainly Meriones libycus, Gerbillus hirtipes, and the golden hamster, Mesocricetus