A comparative study of the development ofDirofilaria repensin resistent and susceptible strains ofAedes aegyptiwas performed to acquire more information about the ultrastructural events in the Malpighian tubules and hemocoel of the mosquito, and to investigate the defense mechanisms activated by the host. Larval growth in the primary cells of the Malpighian tubules of the susceptible strain, associated with increasing changes in cytoplasmic organization of the host‐cell leading to cell death, were observed by scanning and transmission electron microscopy. When the parasite reaches the L3 stage and migrates to the hemocoel, hemocytes rarely intervene and seem ineffective. In the resistent strain, it was observed that the destiny ofD. repensis decided in the Malpighian tubules where the intracellular development of penetrating microfilaria is blocked by chemical and/or biochemical defense mechanisms activated by the host.