During a study of the parasites attacking the forest tent caterpillar, Malacosoma disstria Hbn., a ready means of identifying puparia of the common dipterous parasites proved necessary so that those individuals that failed to develop beyond the puparial stage in laboratory rearings might be identified in advance of lengthy cold treatment and incubation. The following key was therefore developed on the basis of the study of series of puparia associated with adults determined by the Entomology Research Institute. The key includes the ten most common dipterous parasites of the tent caterpillar in Ontario, seven of which have been included in a similar key to the parasites of Malacosoma spp. in British Columbia (Ross 1953). The terminology used in these two keys is similar, and is adapted from Green (1921).