One of serious problems arising in cattle breeding during summer pasturable period is telasiosis, under the disease which additional weights and milk yield of milk decrease, and in started cases (owing to conjunctivitis, keratoconjunctivitis and cornea ulcer) animals lose sight and for this reason are discarded from the herd. The purpose of the researches was studying the occurrence of telasiosis and specific structure of telasiosis in vari-ous municipal regions of the forest-steppe zone of Northern Trans-Urals. For life diagnostics of telasiosis N.M. Gorodovich's data processing method on hel-minthes (1966) was used with the indicator of exten-siveness of the invasion. For studying of distribution of telasiosis among the cattle clinical examination of 19 173 heads was performed during summer period of 2002-2016 with confirmation of the diagnosis microsco-py of washouts from conjunctival cavities of clinically sick animals and detection of larvae of Thelazia. Aver-age long-term extensiveness of invasion of cattle telasiosis in the forest-steppe zone made 11.21±0.82 % and fluctuated slightly depending on a subband. So, in the subband of the northern forest-steppe this indicator made 11.45±1.04 %, and in the subband of the south-ern forest-steppe it was 10.49±0.85 %. Activators of telasiosis in this subband were two types of Thelazia, dominating among which was the type of Th. gulosa - 87.55±2.05 % (from general collecting) in the smallest quantities met a type of Th. skrjabini - 12.45±2.05 %. Essential differences in cattle invasion in various years of research are established. So, the minimum indicator of extensive invasion made 2.53 % in 2002, and maxi-mum - 36.17 % in 2006.