The sex ratio of loach fishes from the warm thermal spring Baile Episcopesti in western Romania was investigated. In Cobitis elongatoides, the sex ratio was balanced, while in Sabanejewia balcanica, males made up 77% of the adult fish. In both species, body size at onset of maturation and maximum length were remarkably smaller than in other populations. Mature males of C. elongatoides measured 35 to 51 mm in total length while the largest female reached 69 mm. Males of S. balcanica measured 40 to 53 mm and the largest female had a total length of 54 mm. We consider both the dwarfism of the fish as well as the male-biased sex ratio in S. balcanica to be caused by the thermal regime in the spring.