Abstract: In order to inquire into the basic rules of the species richness and fauna of fleas along latitudinal gradient and the main ecological factors affecting their distribution, 153 flea species, belonging to 45 genera and 9 families, were collected in mountainous areas at northern latitudes from 21° to 28° in the Hengduan Mountains of western Yunnan, China. All the data were analyzed synthetically with statistics. The results showed that: (1) The richness of flea families, genera, species, endemic species, ratios of endemic species and species of different faunal realms increased first and then decreased with increasing latitude, peaking at 25° to 27°N. The unimodal pattern may be the result of an edge effect between two faunal regions. (2) The horizontal patterns of Oriental and Palaearctic elements were significantly different. A gradually decreasing tendency appeared in Oriental elements with increasing latitude, whereas there was an increasing tendency in Palaearctic. The transitional zone of the two realms ranged from 23º to 29º N and formed an overlap zone and divergence center at 25º to 27ºN. (3) Three main ecological types were identified in the Hengduan Mountains based on cluster analysis. The results reflected the influence of latitude, climate and vegetation on species distribution and community compositions, and also the concordance of flea species distribution and latitu-dinal gradient. (4) The horizontal distribution of β diversity showed two peaks, indicating the transition and transformation of flea species composition and distribution along different gradients, and thus was associated with climate and environmental change. (5) The horizontal patterns of species richness and fauna were simi-lar to the vertical pattern in the Hengduan Mountains. (6) The zones from 25º to 27ºN were the center of dif-ferentiation and overlap of the two realms. Because of the edge effect and complex landscape, the families, genera, species and endemic species of fleas here all showed high diversity, suggesting that this may be the core areas for diversity conservation, distribution and differentiation in China. We conclude that the main factors affecting flea diversity in the Hengduan Mountains are the edge effect in this transitional area of fauna and flora, and heterogeneity of climate, geographical condition and vegetation.