Abstract: An investigation on the fauna and ecology of fleas in the Cangshan Mountain and Erhai Lake Nature Reserve of Dali was carried out from 1992 to 1997.A total of 3 241 individuals belonging to 39 species,21 genera and 5 families was collected.The species diversity,evenness and ecological dominance indices of fleas communities in 7 main ecological environments were measured and compared.According to the investigation,the stability of flea communities is the highest in the coniferous and broadleaf mixed forest and the lowest in cultivated area.The vertical distribution of 39 species of fleas was investigated on both eastern and westsern slope of Cangshan Mountain.21 species (53.84%) belong to Palaearctic realm,16 species (41.03%) belong to Oriental realm,and 2 species (6.06%) are wide spread species.The elevation of 3 100 m (in Cooltemperate coniferous and broadleaf mixed forest) was the dividing line between the two realms in Cangshan Mountain.The authors cosider that delimiting the boundary line between Palaearctic and Oriental Regions in Hengduan Mountains of Southwestern China should base on the altitude,and the boundary line corresponds to the distributional band of the Cooltemperate coniferous and broadleaf mixed forest in this area.