An average of 10% of the rootlets of nonmycorrhizal jack pine seedlings transplanted to four locations in the Canadian prairies became mycorrhizal after one growing season. When inoculated with a mixture of mycorrhizal root tips and forest humus, an average of 35% of the rootlets of seedlings transplanted to the same areas became mycorrhizal. In the greenhouse, mycorrhizae developed only on seedlings grown in soil from forested areas or in prairie soil inoculated with mycorrhizal root tips and forest humus. The addition of forest inoculum is required for satisfactory development of mycorrhizae during the early seedling stage in prairie soils, if regeneration is by direct seeding or if mycorrhization with forest fungi is not assured in the nursery.