Hyalella azteca is a species complex distributed in North, Central, and northern South America. The identity of the species has always been a problem, especially because the original description by Saussure (1858) from a "cistern" in Vera Cruz, Mexico, is poor, and the figures are not clear. Since then, mention of the type material or specimens from the type locality has not been made by investigators using the name H. azteca. Ecological and genetic information available today suggests that there are several species in the complex commonly referred to as H. azteca. The subtle morphological differences among the populations have made the problem of defining these species very complicated. To aid in this process, we present here the morphological description of H. azteca based on the syntype series established by Saussure and deposited in the Museum d'Histoire Naturelle, Ville de Geneve, Switzerland.