The ability of arthropod vectors to support the development and subsequent transmission of two or more species of viruses or protozoa simultaneously is well-documented in the literature, but comparable observations on filarial parasites are lacking. The purpose of this study was to evaluate the ability of a laboratory mosquito, in this case, a filaria-susceptible strain of Aedes aegypti, to support the development of two filarial parasites, Brugia malayi and Dirofilaria immitis, concomitantly, and to assess the effects of these infections on the mosquito vector. Mosquitoes which fed on blood containing 50 microfilariae per 20 cmm of one or the other species ingested an average of 12 microfilariae of B. malayi or 14 microfilarie of D. immitis. Approximately 70% of microfilariae ingested migrated to their appropriate site of development. Dissections of mosquitoes at 9 to 14 days post-feeding yielded an average of 7 to 8 third-stage larvae per mosquito, indicating that most of the microfilariae which migrated from the midgut completed their development to the infective stage. Upon developing to the third stage, the larvae of D. immitis migrated directly from the Malpighian tubules to the head and mouthparts, whereas B. malayi larvae apparently entered the abdominal hemocoel for 1 to 2 days before completing the migration. Loss of third-stage larvae from the mouthparts of mosquitoes feeding on 10% sucrose solution was negligible. Mosquitoes which ingested blood containing 50 microfilariae of each species per 20 cmm yielded an average of 6.5 larvae of B. malayi and of D. immitis. Similar numbers of larvae were recovered from control mosquitoes which ingested blood with 50 microfilariae of one species per 20 cmm. Furthermore, measurements of larvae from mixed and single infections revealed no significant differences in the size of either species of filaria, suggesting that growth processes were not altered. The deleterious effects of mixed infections on the vector were assessed by comparisons of mean daily mortality among mosquitoes harboring either one or both species of filaria. Mosquitoes harboring both filariae experienced mortality (2.32% per day) approximately equal to the cumulated mortality of the 2 groups harboring B. malayi only (.84% per day) and D. immitis only (1.58% per day). The results of the study indicate that there was no interference with development of 2 species of filaria within the same mosquito vector, and that the mortality of mosquitoes resulting from mixed infection was no higher than would be expected given the additive deleterious effects of infection with each species of filaria