IntroductionFour species of Capillaria were found in eight species of wild carnivorous mammals (286 carcasses) taken in Ontario, Canada. C. plica was found in the urinary bladder of raccoon (Procyon lotor), red fox (Vuipes vulpes), coyote (Canis latrans), fisher (Martes pennanti) and striped skunk (Mephitis mephitis); C. putorii in the stomach and intestine of short-tailed weasel (Mustela erminea), mink (M. vison), fisher, marten (Mattes americana), striped skunk and raccoon; C. aerophila in the mucosa of the trachea and bronchi of red fox and marten and C. procyonis in the epithelial lining of the oesophagus of raccoon and striped skunk. All four Capillaria spp. are redescribed and specimens of each species from different host species are compared. A key to the identification of the above species as well as C. didelphis and (7. hepatica is presented. Specimens of C. erinacei and additional material of C. putorii were obtained on loan. Specimens named C. mustelorum by previous authors were collected from carnivores in Ontario. C. mustelorum and C. erinacei are considered to be synonyms of C. putorii.The genus Capillaria is briefly reviewed historically and the recognition of other genera, based on single characters, is discussed and rejected.The possibility of accommodating the Capillaria spp. from Canadian carnivorous mammals in the sub-genera Capillaria and Thominx is discussed and dismissed.t Present address: attempted to split the genus Capillaria by placing species in other genera of which the following have been recognized in recent papers;