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A new species of the freshwater planktonic copepod genus Hesperodiaptomus is described from material collected in several water bodies in the state of Morelos, central Mexico. Hesperodiaptomus morelensis n. sp. differs from its congeners in having a very reduced endopod on the male fifth right leg, and a hook-shaped cuticular process on the inner margin of the first exopodal segment of the same leg. Besides other characters of the female fifth legs, the new species has a distinctive genital somite, with asymmetrical lateral wing-like processes. This species seems to be closely related to H. kiseri mainly because of similarities between the male fifth legs and the structure of the female genital somite. This predominantly nearctic genus is known to be distributed mainly in north-western and central North America, although a few species have been recorded in north-eastern Asia (Siberia). Hesperodiaptomus has not been recorded previously south of California. Apparently, this genus radiated in North America and spread in different directions. The apparent disjunct distribution of at least two species (including the new one) could be explained as resulting from a previous extensive radiation related to glaciation and consequent post-glacial repopulation that might have left these forms as stranded remnants, or by dispersal processes: central Mexico is connected with North America by two high-altitude potential passages. The new species seems to be restricted to a high altitude area in Mexico. It seems unlikely that species of Hesperodiaptomus will be recorded further south on the American continent. Inglés