The study of populations of animals has received the attention of a large number of workers. Of these, however, but a few have been interested in the problem in its application to parasitic nematodes and in this field there is consequently a limited literature. Particularly is this so when the parasitic nematodes of sheep are considered, the incentive for study of which, in most cases, has been the pressing need for their destruction. This desire has been fulfilled to a greater or lesser degree, but at the expense of researches which would have contributed to a more fundamental knowledge of the ecological relationships of parasitic worms in general.