Rotifers are the most diverse group in freshwater zooplankton and play an important role in food webs and ecosystems. DNA barcoding has become a useful approach to investigate species diversity at local and regional scales, but its application is still limited by efficient primers for the group. To test a pair of primers 30F/885R recently designed for rotifers, we applied them to investigating regional species diversity in the freshwater of South China. We sequenced the cytochrome c oxidase

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... nit I (COI) gene of rotifers collected from the investigated 23 reservoirs in a large river basin and obtained 145 COI sequences from 33 species in 14 genera. The mean PCR success rate for all tested species was 50%. The 145 sequenced mtCOI in this study covered 33 of 64 identified morphological taxa, including most of the common species in the basin. The intraspecific genetic distance was calculated with a K2P model for 24 rotifer species occurring in the quantitative samples, in which 15 rotifers, such as Keratella cochlearis and Brachionus calyciflorus, had a genetic distance higher than 5%. The high intraspecific genetic differentiation indicates that cryptic species are probably common in (sub)tropical China.