Due to their interesting biology, conspicuous sexual dimorphism and the ability to conduct experiments on species that breed under laboratory condition, sepsid flies (Diptera : Sepsidae) are becoming increasingly important model organisms in evolutionary biology. Accurate species boundaries and well supported phylogenetic hypotheses are thus of interest to many biologists. Here we resolve the conflict surrounding the taxonomic status of the European Sepsis nigripes Meigen, 1826, which is shown to be a valid species using morphological and molecular data applied to multiple species concepts. The species is also placed onto a phylogenetic tree for the genus Sepsis that includes most European and North American species. In addition, we assess the genetic variability between two populations of the Holarctic Sepsis luteipes Melander & Spuler, 1917 from Europe and North America and find conflicting evidence between morphology and DNA sequences. Different species concepts here yield different inferences, and if two species were to be accepted based on molecular data, Sepsis helvetica Munari, 1985 from Europe would have to be resurrected from synonymy. We provide high-resolution images for all species in order to aid in accurate identification. Both species are also added to Sepsidnet, the digital reference collection for Sepsidae ( http://sepsidnet-rmbr.nus.edu.sg). Lastly, we discuss a field site in the Swiss Alps where 12 species of Sepsis occur sympatrically on the same pasture.