Hippoboscid flies were found on 62 (17%) of 382 northern spotted owls (Strix occidentalis caurina) captured between April and September, 1986 through 1990. Two species of hippoboscids were identified: Icosta americana and Ornithomya anchineuria. Male and female adult spotted owls had similar prevalences and relative densities of hippoboscid flies. Juvenile owls had lower prevalence and relative densities than adults. There were no significant differences in mean intensity of hippoboscids on adult male, adult female and juvenile spotted owls. Relative densities of flies infesting adult owls were significantly greater during years of increased fall temperatures, decreased winter precipitation, and decreased summer temperatures.