The presence of Hippobosca equina strongly influences the behaviour and health status of horses. The aim of the study was to analyse the invasion dynamics of the forest fly in Polish primitive horses (Equus caballus gmelini) in the Popielno Forest Reserve (northeastern Poland). Observations were carried out from May to September, every two weeks for two consecutive years (2012 and 2013) in five study groups: in 11-12-year-old mares with foals, 6-7-year-old leading stallions, 1.5-year-old fillies, 1.5-year-old colts and 9-10-year-old working geldings. Adult H. equina were counted in the preferred sites of their occurrence. The highest prevalence was observed from mid-June to the end of July, at average air temperature was 21°C. Forest flies most frequently affected working geldings, leading stallions, and 1.5-year-old colts. The highest intensity of infection was found in working geldings (28 to 34 insects per animal), in 1.5-year-old colts (10 to 16 insects) and in mares with foals (4 to 14 insects).