This study aimed to determine species composition, diversity, and distribution of rotifers in natural waters in order to select potential rotifer species that can be cultured for biomass to serve as live food for larviculture of economically important species toward diversifying aquaculture species in the Hau River, Vietnam. Sampling was carried out at 10 sites on the mainstream along the Hau River starting from the upstream (An Giang) to midstream (Can Tho, Hau Giang) and downstream (Soc Trang). The collection of the samples included water quality parameters and zooplankton including rotifers (Rotifera). The results showed that a total of 66 species of 31 genera were recorded in Hau River. The most commonly encountered species with high occurrence frequency in Hau River were Polyarthra vulgaris, Brachionus rubens, B. calyciflorus, Filinia terminalis, Keratella cochlearis, K. valga and B. caudatus. In Hau River, 57 species were found during the rainy season and 51 species in the dry season. A difference in species number was detected between sampling locations. The highest species number was found in the midstream areas, with 47 species recorded in the rainy season and 42 species in the dry season, followed by the upstream area with 39 species in the rainy season and 27 species in the dry season. The lowest species number was found in the downstream areas. The mean density of zooplankton was 15,895± 13,901 ind/m³ in which rotifers
accounted for 55.45% (8,815±10,743 ind/m³). There was no significant difference in the density of zooplankton and rotifers between sampling areas during the rainy season. However, in the dry season, Thot Not had much higher
zooplankton density than O Mon, Tra Noc, Ninh Kieu, and Cai Con, and significantly higher rotifer density than Chau Doc, Cai Con, and Tran De.