The effect of habitat nutrients and Ascogregarina taiwanensis (Lein & Levine) infection on the vector competence of a New Orleans strain of Aedes albopictus (Skuse) for Dirofilaria immitis (Leidy) were evaluated. Larvae were infected with A. taiwanensis oocysts and reared under high and deficient nutrient conditions using leaf litter as a food source. Ascogregarine-infected and uninfected females were fed on D. immitis-infected blood and examined after 15 d for filariae. Susceptibility to infection with filaria was 60-70% for all females. In groups reared under high nutrients, the infective rate (3rd-stage larvae present) of coinfected females (18%) was significantly greater than females infected only with heartworm (6%). Host mortality following blood meals was significantly less in coinfected (22%) than in heartworm-infected females (37%). Under deficient nutrient conditions, there was no significant difference between the infective rate (8%) or post-blood meal mortality (5%) of coinfected females compared with heartworm-infected females. Prevalence of melanization reactions in coinfected females was significantly higher (31%) than in females infected only with heartworm (6%) at both nutrient levels. It is concluded that high nutrient levels and ascogregarine infection increase the vector competence of Ae. albopictus for D. immitis by enhancing the immune response so that fewer filariae develop, causing lower host mortality. Under low nutrient conditions, the smaller host contains less food reserves for filariae development, and in coinfected females melanization reactions and damage to the Malpighian tubules may reduce vector survival.