\received DD MMMM YYYY \acceptedDD MMMM YYYY In order to clarify the prevalent species of Gasterophilus (Diptera, Oestridae) in Mongolian horses from different regions and their phylogenetic relationships, A total of 215 third-instar larvae (L3) of Gasterophilus sp. were collected from slaughterhouses in Hohhot, Erdos, Tongliao, and Hulunbeier, the habitats of Mongolian horses in China, in January 2023. Morphological observations were conducted using light microscopy and scanning electron microscopy, and the mitochondrial cytochrome b (cytb) gene was extracted through PCR. For the amplified cytb gene of Gasterophilus sp. from Mongolian horses, relevant gene comparisons were conducted using BLAST in GenBank.Using MEGA6.0, a Clustal W comparison was conducted on the cytb gene of Gasterophilus sp. from Mongolian horses and the homologous cytb sequences of 10 related species from GenBank, and a phylogenetic tree was constructed. The results indicate that the distribution of dorsal spines on the pseudocephalon and the characteristics of the oral cristae of the third-instar larvae of Gasterophilus sp. are consistent with the morphology of the third-instar larvae of Gasterophilus pecorum. BLAST results showed that the cytb gene of Gasterophilus sp. from Mongolian horses had a high identity score of 96.63-99.34% with Gasterophilus pecorum (NC029812) from GenBank. The phylogenetic tree shows that the mitochondrial cytb genes of Gasterophilus sp. from Mongolian horses are separated from 6 other Gasterophilus species in GenBank, as well as those of other species of Hypodermatidae, Cuterebridae, and Calliphoridae. In conclusion, the Gasterophilus sp. collected from Mongolian horses in this study was identified as Gasterophilus pecorum based on morphological and molecular data. The phylogenetic tree shows a certain genetic distance between Gasterophilus sp. reported in GenBank and the one identified in this study.