To assess the spatial variability in external morphology as well as sexual dimorphism of a non-native population of topmouth gudgeon Pseudorasbora parva within an ontogenetic context, triple regression analysis (distance-based measurements) and statistical tests were applied to data from a heated lake in Poland. Moreover, this population just invaded this extreme and thus special habitat in 2003, and therefore it represents a novel environment and its first stages of establishment. It has often been reported that topmouth gudgeon is very flexible when reaching new environments and this also shows in forming different phenotypes. We found that mature males and females have not only a different morphology, but also the development of the traits is different. These findings correspond with the specifics of this particular type of habitat.