To determine whether age of the microfilaria influences its ability to develop in the mosquito host, an uninfected dog was transfused with blood from another dog showing about 30,000 embryos of Dirofilaria immitis per cm3 of blood. It was calculated that about 5400 microfilariae per cm3 of blood should be found in the recipient after transfusion. Anopheles quadrimaculatus females were fed upon the recipient at various times for 4 mos. after the transfusion, and normal development of the parasite in the mosquito was observed for 3 mos. Encapsulated 3rd stage filarial larvae were found in the mosquitoes only during the 1st mo. after transfusion. Additional observations were made with very young microfilariae obtained from the uteri of D. immitis adult females and females of the frog filaria, Foleyella brachyop-tera. These were fed through membranes to Aedes aegypti and A. quadrimaculatus which were dissected 12 to 15 days after their infective meal. None of the D. immitis embryos developed in the mosquitoes and very few F. brachyoptera developed. This suggests that the embryos may have to undergo a short aging period before they are fully infective to the mosquito host.