The paper presents the results of a faunistic investigation on the genus Sympetrum in two regions in the SW part of Romania-Timiș Plain (Banat region) and Oltenia region. The faunistic account comprises four species: Sympetrum fonscolombii (Selys, 1840), Sympetrum striolatum (Charpentier, 1840), Sympetrum sanguineum (Müller, 1764) and Sympetrum meridionale (Selys, 1841). The genitalia of the collected species are compared with data from the available literature. Following, an analysis of the European Sympetrum species is provided from the perspective of the classical principles of systematic zoology: the biological species concept, the reproductive isolation of species in Odonata, species delimitation, speciation.