In this study, polymorphism in behaviour, morphology, and stable isotope signatures of burbot from Lake Southern Konnevesi, Finland, Europe, was examined. First, local knowledge was collected on exceptional polymorphism of the spawning behaviour and morphology of burbot. These phenomena were then studied based on catch samples. Interviews and catch sample analyses suggested two morphs of burbot: one morph spawning in late February in the littoral zone and other in late March, in deep profundal, depths of about 30 m. Fish caught from the profundal zone had higher average proportional somatic body weights and wider heads than those caught from the littoral spawning sites in February. The length-at-age of the individuals from the littoral catch was longer than that of the profundal catch. Stable isotope analysis revealed differences in the mean carbon isotope ratio between sampling sites, suggesting differences in diet. Variability in several of the aforementioned variables was correlated. This study is the first to imply polymorphism in burbot populations from a lake in Eurasia, where burbot are considered as near-threatened species.