This paper provides a revision of the extinct Mesozoic family Prochydoridae Smirnov, 1992 (Crustacea: Cladocera). Representatives of the two previously described genera, Prochydorus Smirnov, 1992 and Archeoxus Smirnov, 1992, are re-investigated from Khotont (Mongolia, Jurassic–Cretaceous boundary). A new genus, Palaeorak gen. nov., is described from Khasurty (Asian Russia, Lower Cretaceous). The study provides evidence that the three known prochydorid genera show a subfamilial or familial level of morphological differentiation. Evidence is presented that the Prochydoridae form a specific lineage of cladocerans either separate from or basal to the order Anomopoda Sars, 1865. Nevertheless, a definitive conclusion of phylogenetic position awaits description of the prochydorid thoracic limbs, which are currently unknown. © 2009 The Linnean Society of London, Zoological Journal of the Linnean Society, 2009, 155, 253–265.