The survey on the thelazian parasites and their intermediate hosts was made in the farm of National Institute of Animal Industry in Chiba city from January 1972 to May 1973. Four genera and 6 species, totalling 9,097 flies were obtained from cattle bodies and Musca hervei was dominant (63%). Among the collected flies, some of Musca hervei and Musca bezzii were infected with thelazian larvae, and 93% of them were females of M. hervei. Interest result of this survey was a discovery of thelazian larvae in males of M. hervei. Krastin (1950) supposed that the larvae invade into the follicle of ovary and develope therein. However, Vilagiova (1962) found larvae of T. gulosa in fat tissue of male and ovarian follicle of female in M. autumnalis and supposed the similar relationship also in T. rhodesi and T. skrjabini. The authors could not observe the larvae in the follicles, but found tunicate larvae adhering to the wall of body cavity in both sexes of M. hervei. On the origin of this tunica is a problem in future. Month of maximum infected rate both in the flies and cattle eyes was October. The transmission of thelazian larvae to the eyes of cows by flies was continued from spring until late fall. Adult parasites are kept in the eyes of cows during the winter and new infection of larvae to the flies begins at the same time with emerging of the overwintered flies. New infection of infectious larvae to the eyes of cows begins in the middle of May, because newly infected larvae from cows to flies in early spring take more than 50 days to develope to the infectious larvae.
