By placing parasite-free calves in paddocks grazed by infected animals for 18 day periods at various times during the previous season it was shown that eggs of Ostertagia ostertagi, Cooperia oncophora and Nematodirus helvetianus deposited on pastures from early July to October of one year were able to survive in the Maritime area of Canada over winter either as eggs and/or larvae and contribute to residual infections on these pastures the following spring. The greatest deposition and/or survival of those eggs that were shed on pasture occurred in August and September for Cooperia and in September and October for Ostertagia. Greatest deposition of Nematodirus occurred in July and August and relatively few Nematodirus eggs shed in late September or early October were infective early in the next season. The number of generations of worms per year was low, ranging from one to two or perhaps three per year depending on the species. There was a delay in the maturation of many worm eggs. Residual overwintering infections play a significant role in the establishment of initial infections each summer in susceptible stock. These animals recontaminate the pastures leading to the subsequent development of large numbers of infective larvae by late summer and autumn.