Abstract: Objectives of this study were to analyze morphological characteristics of third instar larva of Gasterophilus pecorum and behavior of adult flies by laboratory culture and field observation from 2008 to 2010. G. pecorum larva parasitize inside bodies of local domestic horses that live sympatricly in the strictly protected area with reintroduced Przewalski horses (Equus ferus przewalskii) in Kalamaili of Xinjiang. A total of 920 third instar larva were collected and cultured, of which 35 (2.9%) individuals pupated successfully and pupal period of G. pecorum varied from 13 to 27 days with a peck between 18th and 20 th days (42.86%). 17 (48.57%) individuals emerged and emergence period varied from 15 to 27 days with a peck between 21 th and 22 th days (22.86%), of which 11.76% were males and 88.24% were females with a sexual ratio of 0.13. Their longevity varied from 2 to 8 days, but the higher frequency was found from 4 to 6 days (58.82%). It was observed that length, width and weight of larvae bodies were (13.27.0) mm, (5.14.0) mm and (24130) mg, respectively. However, correlation analysis demonstrated that the length, width and weight of larvae bodies did not correlate with sex of adult flies. Results obtained from field investigation showed that mating behavior of G. pecorum was not observed in the wild field or near the horse. Host seeking and grooming behavior at the early emergence period was found. The eggs of G. pecorum were not found on the host surface and stems or leaves of plants.