In this paper the authors consider sex ratio in populations of the bloodsucker of the genus Ornithomya Latreille located in the Vladimir and Murmansk Regions. During the ringing of birds since 2014 we have been collecting the bloodsucker flies parasitizing on them. For just four years in the Vladimir Region we have examined 12515 birds, from which 94 females and 17 males of the bloodsucker Ornithomya avicularia L., 1758, as well as 142 females and 37 males Ornithomya fringillina Curtis, 1836,

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... re taken. On the territory of the Murmansk Region in 2016 and 2017 465 females and 91 males of Ornithomya chloropus Bergot, 1901 were collected from birds. As a result of our research we obtained and summarized original material on the composition of the sexual structure of the population in bloodsucker flies (Diptera, Hippoboscidae) belonging to the genus Ornithomya Latreille, 1802. Females dominate males in Ornithomya avicularia , Ornithomya chloropus and Ornithomya fringillina in populations located on the territory of the Vladimir and Murmansk Regions of the Russian Federation. The study of seasonal changes in the sexual structure of the above species showed a gradual decrease in the number of male individuals during the season.