Laboratory studies on the immune response of Aedes trivittatus to Dirofilaria immitis revealed an active immune response to developing larvae. A response was noted in 41 of 268 (15.3%) mosquitoes that fed on an infected dog. Mosquitoes exhibiting an immune response contained a significantly greater number of parasites (21.9 larvae/mosquito) than mosquitoes in which no response was evident (14.7 larvae/mosquito). The microfilarial stage was melanized more frequently than later developmental stages, although all developmental stages were affected. Melanized microfilariae appeared to be voided from the Malpighian tubules through the hindgut and anus. Early melanization was directed at the area of the excretory pore of the parasites, suggesting at least an indirect response to excretory products. Microfilariae that accidentally migrated through the Malpighian tubules into the haemocoele were completely encapsulated and melanized. All microfilariae that were intrathoracically inoculated into the haemocoele were completely encapsulated and melanized by four days after inoculation. The role the immune system might play in limiting parasite burdens of D. immitis in A. trivittatus is discussed.