Reptiles show a remarkable diversity of sex determination mechanisms and sex chromosome systems, derived from different autosomal pairs. The origin of the ZW sex chromosomes of <i>Lacerta agilis</i>, a widespread Eurasian lizard species, is a matter of discussion: is it a small macrochromosome from the 11-18 group common to all lacertids, or does this species have a unique ZW pair derived from the large chromosome 5? Using independent molecular cytogenetic methods, we investigated the karyotype of <i>L. agilis exigua</i> from Siberia, Russia, to identify the sex chromosomes. FISH with a flow-sorted chromosome painting probe derived from <i>L. strigata</i> and specific to chromosomes 13, 14, and Z confirmed that the Z chromosome of <i>L. agilis</i> is a small macrochromosome, the same as in <i>L. strigata</i>. FISH with the telomeric probe showed an extensive accumulation of the telomere-like repeat in the W chromosome in agreement with previous studies, excluding the possibility that the lineages of <i>L</i>. <i>agilis</i> studied in different works could have different sex chromosome systems due to a putative intra-species polymorphism. Our results reinforce the idea of the stability of the sex chromosomes and lack of evidence for sex-chromosome turnovers in known species of Lacertidae.