Diaphanosoma is a dominant genus of cladocera in South China, and widely distributes in lakes and reservoirs.To un-derstand its regional population distribution, the animals were sampled in dry and flood seasons in 19 reservoirs of Guangdong Prov-ince during 2001 and 2006 monthly or seasonally in three reservoirs of Feilaixia, Xinfengjiang and Dajingshan, which have been under different trophic states.Adult Diaphanosoma was found in all reservoirs, where 5 reservoirs had only D.dubium, 8 reser-voirs had only D.orghidani, and 6 reservoirs had the two species.The two species showed an apparent difference in niches.D.dubium mainly dominated in eutrophic reservoirs, while D.orghidani did in oligotrophic reservoirs.Distributions of D.dubium and D.orghidani in the reservoirs can be well explained by their differences in morphology, food selection and ability of avoiding predators.