In order to clarify the species status of the Corsican endemic Polyommatus (Lysandra) coridon nufrellensis we carried out experimental hybridisations with the nominotypical subspecies from Germany, because these can be informative about the existence of reproductive isolating mechanisms. Additionally, molecular analyses were done to resolve the degree of differentiation between both taxa. Six nufrellensis females could be mated to males of the nominotypical subspecies with the semi-artificial method. These produced males and females of the F₁ hybrid generation. One mating could be achieved amongst these hybrid butterflies. This mated female laid a very large number of eggs (433), most of which turned out to be fertile and produced 24 males and 19 females of the F₂ generation. Adults of both the F₁ and the F₂ generation had characters of both parental taxa. An analysis of DNA sequences from the mitochondrial (COI) as well as the nuclear genome (ITS2) with statistical parsimony networks did not provide evidence for genetic differentiation of nufrellensis from the nominotypical subspecies. Our results point to a young and incomplete speciation process, and therefore we suggest keeping the current taxonomic treatment of nufrellensis as a subspecies of P. coridon. As an addendum to part II of our series on the coridon populations of the islands of Sardinia and Corsica we report the successful breeding of the F1 hybrid generation of P. c. gennargenti × P. c. nufrellensis to the F₂ generation.