Two new chydorid cladoceran genera, Planicirclus (n.gen.) with one species Planicirclus alticarinatus (n.sp.), and Plurispina (n.gen.) with two species, Plurispina chauliodus (n.sp.) and P. multituberculata (n.sp.), are described from south-western Australia. The status of the chydorid genus Pleuroxus in southern Australia and New Zealand is reviewed, with two new species described: Pleuroxus foveatus (n.sp.) from south-western Australia, and P. helvenacus (n.sp.) from New Zealand. The European species Pleuroxus aduncus (Jurine), previously identified from the region, is described, and shown definitely not to occur there. Pleuroxus hastirostris Sars and P. inermis Sars are redescribed, as is Archepleuroxus baylyi Smirnov & Timms. Four other species of Pleuroxus described from southern Australia are considered nomina dubia. All eight recognised taxa appear to be endemic to the region. Marked differences between the Pleuroxus fauna of eastern and western Australia provide evidence for adaptive radiation. It is likely that a comparable high degree of endemicity will be found in other chydorid groups.