Filarioid worms (Dirofilaria immitis, D. tenuis, Dipetalonema procyonis, and D. llewellyni) were discovered in raccoons (Procyon lotar) in Maryland. Raccoons were trapped in lowland, upland, and agricultural-residential areas, which were further classified as stream borders, poorly drained, and well drained. Data on incidence of D. llewellyni were analyzed on basis of host distribution within these areas to indicate type of habitat in which one might seek the vector. It was concluded that exposure takes place in the spring of the year. The arthropod found associated most often with the raccoon in spring was Ixodes texanus. Larvae of this tick which were fed on infected raccoons presented no evidence of development of the microfilariae. Feeding experiments were also conducted with mosquitoes: Aedes aegypti, A. canadensis, A. sollicitans, A. triseriatus, A. vexans, Culex pipiens, Anopheles punctipennis, and A. quadrimaculatus. Although microfilariae remained alive and active in the gut contents of all these mosquitoes for 2 days, only in Aedes aegypti did they enter the hemocele, but no developmental changes were noted and all microfilariae were dead by the eighth day. Although the intermediate host of D. llewellyni was not determined, evaluation of the accumulated data provides criteria for seeking the vector. It appeared unlikely that exposure of the raccoons took place in the den or that the filarioids were transmitted by an ectoparasite commonly found in raccoon dens. The data suggest that the vector is available only early in spring, although there are infected raccoons throughout the year. Prevalence in juveniles was 21 percent; in subadults, 64 percent; in adults, 87 percent.
