Rotifers were sampled monthly from February to November in 2009 in Lake Erhai, Yunnan Province, China, in order to study their diversity and seasonal dynamics. Lake Erhai has a rotifer composition with tropical-subtropical characteristics:68 species belonging to 25 genera of 17 families were identified, the majority of which were eurythermal or mesophilic. The genera of Trichocerca, Lecane and Brachionus were more abundant than others, accounting for 37% of the total community. Lecane arcuata and Trichocera inermis are new records in China. Biomass of the rotifer communities in Lake Erhai exhibited a bimodal seasonal cycle. An increase occurred during spring from low winter levels to an annual maximum in the mid fishing moratorium and a second increase in the early fishing season. In the mid fishing moratorium, the rotifer biomass was dominated by Asplanchna priodonta; while in the early fishing season, it was dominated by the small-sized Keratella cochlearis and Polyarthra vulgaris, then by A. priodonta again. The seasonal fishing moratorium affected not only the predation pressure on rotifers, but also competitive pressure on rotifer through influencing the community structure of Cladocera. Through the influence of predation and competition, rotifers were dominated by large omnivorous A. priodonta, which showed a bimodal seasonal distribution pattern.