Development of Dirofilaria immitis (Leidy) in Aedes trivittatus (Coquillett) was followed in the laboratory at 26.5+ 1 C and 80 + 5% RH. Juvenile D. immitis molted from 1st to 2nd stage and 2nd to 3rd stage at 7-8 and 10-11 days postexposure (PE), respectively. Mosquitoes showed a nearly 100% infection rate. Large numbers of microfilariae were retained within the midgut of exposed mosquitoes, and microfilariae invaded the Malpighian tubules through the first 6 days PE. A susceptible dog, exposed to the bite of 49 infective Ae. trivittatus, showed a microfilaremia of D. immitis by about 210 days PE. The role of A. trivittatus as a natural vector of D. immitis is discussed.