Twenty-four female ferrets, approximately 4 mo of age, were subcutaneously inoculated with 60 infective larvae of Dirofilaria immitis to determine migration and development of the worms until they became young adults in the heart and associated vessels. Twelve groups of 2 ferrets each were examined at 3, 5, 11, 15, 21, 49, 56, 63, 70, 91, 119, and 140 days postinoculation, respectively, to recover worms. Total worm recovery from each ferret varied from 1.6 to 79.3%. Worms were found mostly in subcutaneous tissue and muscle from the beginning of infection up to and including day 91. Worms were first recovered from the heart at day 70, when 3.8% of the worms had reached this site. By day 119, essentially all of the worms had migrated to the heart, as indicated by similar overall worm recoveries at 119 (mean 59.0%) and 140 days (mean 65.8%). Some worms remained in the tissues even at day 140. The third and fourth molts occurred as early as days 3 and 56, respectively. On day 91, i.e., soon after the worms reached the heart, the mean lengths of males and females were 58.8 mm (range 43.0-75.0 mm) and 63.3 mm (range 55.0-69.0 mm), respectively. At 140 days, male and female worms from the heart measured 118 mm (range 62.0-146.0 mm) and 144 mm (range 105.0-168.0 mm), respectively.