Methods are presented for the cryopreservation of a sheathed microfilaria, Brugia malayi, and an unsheathed species, Dirofilaria corynodes. The former survived best when frozen at the rate of -0.8° or -0.5°C per minute using 9% dimethyl sulfoxide (DMSO) as the cryopreservative. Approximately 52–79% of the thawed microfilariae developed to the third stage in Aedes aegypti mosquitoes versus 79% of the unfrozen specimens. For D. corynodes the optimum freezing rate was -2° or -5°C per minute, and 6% DMSO combined with 0.004 M polyvinylpyrrolidone (PVP) afforded the best cryoprotective effect. The development of thawed microfilariae in mosquitoes ranged from 22–32% versus 29% for unfrozen specimens. In general, the viability of both species of microfilaria was retained best when stored in liquid nitrogen (-196°C). The entire life cycle of B. malayi was completed in the laboratory using cryopreserved microfilariae as the initial source. The cryopreservation of Wuchereria bancrofti also is discussed.