Comparative morphological investigation of extensive series of Pleuroxus denticulatus Birge from North America and Europe revealed considerable variability in certain characters. These are quantitative (body dimensions), qualitative (structure of postabdomen), and meristic (number of ventro–caudal shell denticles). With regard to the postabdomen, two morphotypes, connected by transitional forms, can be distinguished in both sexes, which, however, are not separated geographically. Nevertheless, there are certain field populations (e.g. from Indiana and southern Germany) that are so strongly divergent in certain characters they would satisfy the taxonomic requirements for assigning them to different geographic races. The North American population s are distinguished by having significantly more denticles on the ventro–caudal angle of the valves, the frequency distribution of which satisfied the 75% rule for the separation of subspecies. However, we do not consider the distinctness in this single character of a single west European population sufficient to justify the establishment of two geographic subspecies. Trunk limb morphology, which yields important conclusions concerning the systematic affinities of Pleuroxus denticulatus, is also described. The species, along with Pleuroxus truncatus (O. F. Müller) and Pleuroxus proauvus Birge, form a group of closely related species, characterized by the sperm ducts opening on the lateral surfaces of the postabdomen and by the relatively small structural differences between the male and female postabdomen. Finally, differentiation of characters within the truncatus group is analyzed in detail. Key words: Cladocera, Chydoridae, Pleuroxus, taxonomy, morphological variation, geographical distribution