An isolate of Trichinella of polar bear origin was studied by isoenzymatic typing. It was found referable to Trichinella nativa. While the Wistar rats proved nearly refractory to this isolate, the Swiss albino mice were highly susceptible. Ninety-one per cent of the cystic lesions in the diaphragm of the polar bear contained viable larvae after over 20 years of acquisition of the infection by the host which is a case of extreme adaptability of the parasite to its host. The anatomo-pathological aspects of these lesions are studied and the zoonotic significance of this isolate examined.