Development in mosquitoes of Brugia malayi microfilariae obtained from the blood of jirds was compared to that of microfilariae from the peritoneal cavity. Penetration of the mosquito midgut wall as well as development into third-stage larvae was assessed. About 70% of blood-borne microfilariae penetrated the midgut wall whether ingested directly from a microfilaremic jird or from a membrane feeder containing blood from the same donor. In contrast, less than 30% of microfilariae from the peritoneal cavity penetrated the midgut wall. Microfilariae in the peritoneal cavity of jirds vary in ability to penetrate the midgut of mosquitoes; some penetrate as rapidly as do blood-borne microfilariae, others penetrate more slowly, and most fail to penetrate the midgut. Regardless of origin, microfilariae that penetrated the midgut wall developed into third-stage larvae.