The ferret M. p. furo, was susceptible to D. immitis infection when exposed to low (14) or high (280-420) numbers of infective larvae harvested from Aedes aegypti. Ferrets (8, half of them cortisonized) were inoculated s.c. with 14 larvae each. All of them were subsequently found to harbor D. immitis in the heart, and all but 1 of them had worms of both sexes. Six of these ferrets were examined for microfilaremia at 31-35 wk after inoculation; 3 were positive (1 observed only at postmortem examination) and there was evidence that fertilization of female worms had occurred in 1 other. Females up to 25.5 cm and males up to 16.0 cm were recovered. There was no evidence that the cortisonization of some ferrets had affected the infections. Both male and female ferrets became infected. Four cortisonized ferrets were inoculated with 280 or 420 larvae of D. immitis (divided equally between s.c. and i.p. routes). All of them died 16-18 wk after inoculation, yielding 102-125 immature D. immitis. In these lethal infections, worms were recovered from the heart and adjoining vessels, and also from vascular and extravascular sites throughout the body.