Third-stage larvae of F. martis entered the gastric glands of mink (Mustela visoi) where they molted on the 3rd or 4th days after infection. Fourth-stage larvae penetrated the lamina propria and entered the submucosa where they were closely associated with the adventitia of blood vessels. Larvae passed through the external muscle layers of the stomach in the adventitia of the small gastric arteries and had molted to the fifth stage in the gastric subserosal fat by 11 days after infection. Subadults passed via the adventitia of the gastric arteries and the pancreatico-duodenal artery to the base of the coeliac axis at its junction with the dorsal aorta. The worms, in the adventitia of the dorsal aorta, moved forward through the diaphragm to the base of the heart. At the base of the heart the worms left the adventitia of the dorsal aorta and transferred to the adventitia of the pulmonary arteries. They moved along the adventitia of the pulmonary arteries and reached the lung parenchyma as early as 15 days after infection. The worms grew markedly in the connective tissue surrounding the arteries and associated bronchi. Finally the tail of the female was forced through the bronchial epithelium. First-stage larvae were deposited on the bronchial epithelium approximately 40 days after infection. F. martis (Werner, 1782), a common nematode parasite of wild mink (Mustela vison) in North America, produces nodules at the hilus of the lobes of the lung. A prevalence of 45% in wild mink was recorded in Ontario by Anderson (1962). First-stage larvae, passed in the feces of the final host, develop in several species of gastropods (Petrow and Gagarin, 1937; Anderson, 1962); the third stage is reached in about 16 to 18 days (Anderson, 1962). Anderson (1962) showed that the prepatent period of F. martis in mink in Ontario is approximately 41 days. He noted that infections of F. martis in mink are often mixed with P. pridhami whose prepatent period is about 20 days. The intramammalian development and migration of F. martis have not previously been studied. The aim of the present work was to determine the route of migration, development, and pathogenesis of F. martis in mink during the prepatent period. MATERIALS AND METHODS Two terrestrial land snails, Mesodon thyroidus and Triodopsis albolabris, were cultured as intermediate hosts. The snails were kept in terraria, the floors of which were covered with moss. The snails were fed commercial cat food, lettuce, and oyster shell. First-stage larvae were recovered from feces by the Baermann technique. Larvae were washed and resuspended to a concentration of 15,000/ml Received for publication 25 November 1969. * Department of Pathology. t Department of Zoology. in a 0.65% solution of sodium chloride containing 1,000 international units of penicillin G potassium and 1,000 fig of streptomycin sulfate per ml. Snails, approximately 2.4 cm in diameter, were injected with 0.2 ml of this larval suspension (approx 3,000 larvae) using a 1-ml disposable tuberculin syringe and a 25-gauge hypodermic needle. The needle was introduced into the snail just anterior to the shell, directed into the posterior area of the foot, and the solution injected as rapidly as possible. The snails were replaced in the terraria for at least 21 days before being used for the recovery of infective larvae. Infective larvae were recovered by digesting the soft tissues of the snails with pepsin hydrochloride at 37 C in a Baermann funnel. Mink were obtained from the colony of the Ontario Veterinary College. Kits were weaned at 6 weeks and vaccinated against canine distemper, viral enteritis, and botulism. They were kept in standard mink cages raised approximately 2 ft from the ground and fed a standard mink ration. Mink were inoculated under ether anesthesia by placing the larvae directly in the stomach via a stomach tube. One mink was killed at each 2-day interval from 1 to 13 days after infection and then at 4-day intervals from 13 to 41 days after infection. Mink were killed with sodium pentobarbital (180 mg/ 2.3 kg body weight). A post-mortem examination was carried out and the following tissues fixed in formalin, sectioned, and stained with hematoxylin and eosin: esophagus, stomach, small intestine, large intestine, mesenteric lymph node, pancreas, kidney, liver, spleen, lung, diaphragm, trachea, heart, brain, and spinal cord.