Abstract: AIM:To investigate the mechanism of Lagenidium giganteum for invading and killing mosquito larvae, the interaction between the fungus and mosquitoes,and histological changes of mosquito larvae infected by this fungus were observed in the laboratory. METHOD: The larvae of Culex quinquefasciatus and Aedes albopictus of different infection periods by L. giganteum were fixed.Their paraffin sections were made and stained with H. E. method, observed with Olympus microscope and photoed with PM 10 auto camera. RESULTS: During the early stageⅠof infection, many fine branchy vegetative mycelia of L. giganteum were found in tissue gap of the head and thorax; later, the mycelia entered the hemocoel and developed to a large number; subsequently, they invaded the muscles,stomach,middle gut, and fat bodies of the host. Some hyphae even grew in cells of these organs or tissues and destroyed them. In stage Ⅱ of infection,remarkable destruction and disappearance of tissues were observed. The vegetative mycelia developed into reproductive form which were septated and were shorter and wider. Then, they became sporangia in round,ovoid or bead string forms. By the end of infection,most of the normal tissues of larvae disappeared, and the cytoplasm and nuclei of mycelia and sporangia of the fungus also disappeared. Only empty tubes derived from the cell wall of the fungus were found in larval bodies. CONCLUSION: It was considered as one of the lethal causes of mosquito larvae that L. giganteum invaded and destroyed cells of tissues and organs, exerting damage consequence on the host.