Morphology of the larval stages of Elaeophora schneideri from the horse fly and the normal definitive host (Odocoileus hemionus) are described. Development of larvae in the horsefly begins in the fat bodies, but larvae enter the haemocoele early in the first-stage of development and remain in the haemocoele until they are third-stage (infective) and then migrate to the head and mouth parts. Infective larvae in the definitive host begin development in the leptomeningeal arteries and are immature adults in 2 weeks. At 3.5-4 weeks they migrate into the carotid arteries and grow to sexual maturity about 4.5 months later. In abnormal definitive hosts (Cervus canadensis or Ovis aries) larvae remain in the leptomeningeal arteries for 5 weeks or longer initiating the clinical manifestations of elaeophorosis.