Details of the body structure of sexually mature copepods (females and males) of two morphologically similar species of the genus Thermocyclops are presented. One was the invader, T. taihokuensis from the Shat Reservoir, Tula Region, while the other the native T. oithonoides from Lake Pleshcheyevo, Yaroslavl Region. Both were studied to clarify the known morphological features and identify new ones so as to reliably distinguish between these species. The structure was analyzed using AxioVision Rel. 4.8 and ScopePhoto 3.0 microscopes. Drawings of the fine structures were made from photographs. The distinctions between these two species are shown to be mainly based on qualitative characteristics (the shape of the element, its location, etc.). No taxonomically significant differences have been found in the measured parameters of the elements of the fourth, fifth, and sixth (in males) legs, caudal rami and their setae, all commonly used to identify species of the genus. The shape of the seminal receptacle, the apical setae of the caudal rami, the apical spines of the endopodite of the fourth legs (P4), and the position of the lateral seta on the caudal rami have been found the most important for identifying the females of these species. Interspecies differences were revealed in the armature of the prominences of the intercoxal coupler, the ornamentation of the P4 coxopodite and its intercoxal coupler, as well as the armature of the 14th segment of the antennules of females. These characteristics are proposed as additional features for species identification. Thermocyclops taihokuensis males are shown to be significantly larger than those of T. oithonoides, and they also differ in the position of the lateral setae of the caudal rami, the P4 armature, and the length of the central apical setae. A key is proposed for identifying six species of the genus Thermocyclops common in European Russia.
