During the last two years the plerocercoids of the broad tapeworm, Diphyllobothrium latum (L. 1758) have been found in fishes from one locality in Michigan, another in Minnesota, and the lake regions of Ontario, Manitoba and Alberta. It has been pointed out1 that nearly 80 per cent of the wall-eyes, Stizostedeon vitreum Mitch., and of the great northern pikes, Esox lucius L., which are consumed in the United States are imported from the large lakes in the three provinces of Canada that have been mentioned. Fish of these species from these lakes are infested with plerocercoids which, when fed to "clean" dogs, have produced typical tapeworms of the species D. latum. Although the number of fish examined from each lake is not sufficient to allow safe generalization, the present data show that the fishes of Lake Nipigon, Ontario, carry the heaviest infestation. It has been possible to account for