The helminths, Meniezia expansa, Ostertagia circumcincta, Nematodirus filicollis and Trichostrongylus colubriformis demonstrated the ability to survive a normal Canadian winter and to cause infection of sheep when ingested during the following spring. Temperatures during the four-month period ranged from −15° F. to 52.5° F. It is concluded that pastures cannot be considered free from these helminths after exposure to such temperatures.