Abstract
The present study of cladocera of the genus Chydorus from South Vietnam revealed a new species, C. idrisi sp. nov. and a rare species, C. breviceps (Stingelin, 1905). C. idrisi sp. nov. belongs to the C. eurynotus Sars, 1901 group, characterized by small labral plate with rounded tip and by absence of honeycomb sculpture on the valves. C. idrisi sp. nov. differs from other species of the group by the sculpture of valves, morphology of rostrum, postabdominal claw, inner distal lobe of first thoracic limb and male postabdomen. Morphology of C. breviceps was studied for the first time. C. breviceps has only one major head pore, and differs from the majority of Chydorus species by numerous other characters. Taxonomic position of studied species is discussed.