We report the species of crustacean zooplankton found in 19 resevoirs and ponds that were sampled at on at least two occasions in Aguascalientes State, Mexico, at a latitude of about 21°30' North Latitude . We collected a total of 33 cladoceran taxa, 15 calanoid and cyclopoid copepods, an Ergasilus copepod, 2 anostracans, one notostracan species and an amphipod . All these taxa had previously been reported in Mexico . As in more temperate areas, there was a significant correlation between the species richness and lake size . The species-area relationship for Aguascalientes was indistinguishable from that of more northerly lakes . The two most common associations of zooplankton species were (1) those small species that occurred with fish : Daphnia parvula, Diaphanosoma birgei, Leptodiaptomus siciloides, Mastigodiaptomus albuquerquensis, and Thermocyclops inversus, and (2) the large Daphnia schodleri that occurred with the predator Notonecta, in the absence of fish . With minor variations, these two associations also occur throughout temperate North America .