Pelophylax esculentus is a vertebrate animal with hemiclonal heredity, attracting the interest of many reproductive and evolutionary biologists. It is a hybrid between P. ridibundus and P. lessonae. These three taxa form the so-called Pelophylax esculentus complex with a population structure usually comprising a hybrid taxa and one parental species. Data on population types at the southernmost distribution area of their sympatry are rare. Here we sampled five sites in inland Croatia in order to analyse the population structure, sex ratio and age structure. The individual genotypes of 93 randomly collected water frogs were verified with allozyme markers for three species-specific polymorphic loci. In order to estimate population age structure, the annual growth rate (skeletochronology) and growth index profiles were also investigated. The growth index profiles were analysed by an estimation of number of lines of arrested growth visible in the cross-section of femur bones. Our results revealed the presence of the R-E-L population with a dominance of P. esculentus. Pelophylax ridibundus was the least abundant taxon but with a relatively high age estimate of eight years on average. Its annual growth rate did not differ from the remaining two species. Gene introgression of mostly ridibundus alleles was also observed in hybrids. Most profiles of gonads in hybrids showed presence of both parental genomes with dominance of ridibundus alleles. The study area represents one of the southernmost distributions of the hybrid taxon in Europe, making it attractive to study gene flow and impact of P. esculentus on P. ridibundus, a typical water frog representative of the Balkan Peninsula.