We explored the population systems of Pelophylax esculentus at the southern border of the distribution range for this complex. We used both morphological data and allozyme markers to describe population systems in this region. All three taxa in the complex, P. ridibundus, P. lessonae and their hybridogenetic form, P. kl. esculentus occurred in the research area. In four localities that were analysed near the Danube River, we found complex population systems of both the parent species and their hybridogenetic form. Southward from the border defined by the Sava and Danube riverbanks, only the P. ridibundus — P. kl. esculentus (R-E) system was detected. Among these R-E populations, the abundance of the hybridogenetic form declines sharply from the north to the south. The most common and widespread system in Central Europe, the P. lessonae — P. kl. esculentus population system, was detected in only one locality near the Sava River.