In Poland, the Aesculapian snake, critically endangered at the edge of its European range, inhabits the lower parts of the Bieszczady Mts. in the SE of the country. This research, carried out between 2009 and 2014, included fieldwork in areas selected on the basis of historical literature data and the results of interviews. 51 breeding mounds were deployed at historical and potential sites of this snake's occurrence on the forested area of 220,886 ha as a supportive tool for detecting these reptiles. The 371 records of the Aesculapian snake obtained are displayed on maps showing its historical distribution in two periods (1958–1989 and 1995–1998) for comparison. Most records were collected in the centre of the species' range in the Bieszczady Mts. Most of the breeding mounds (17 out of 29) in this area were colonized by the snake, whereas only two sites were colonized in the outside. The results show that the Aesculapian snake's range in the Bieszczady Mts has been shrinking. However, most current single records outside the species' centre of occurrence are from localities close to its historical range; this suggests that relict populations may be surviving there. The proportion of records on the breeding mounds grew in consecutive years. This suggests that breeding mounds deployed near the snake's existing localities may be a useful tool for monitoring endangered oviparous snakes and their active conservation.