The ruffe, Gymnocephalus cernuus (L.), is considered to be a food competitor with many commercially valuable fishes such as bream, Abramis brama (L.), roach, Rutilus rutilus (L.), pikeperch, Sander lucioperca (L.), and perch, Perca fluviatilis L. This paper presents the results of an investigation into the fish community structure of Lake Dusia and the population parameters (morphometry, age, growth, diet) of ruffe as one of the prevailing fish species in the lake. The fish community of Lake Dusia was dominated by ruffe, perch, and roach. The ruffe population constituted 64.1% of the fish community of the lake in relative abundance and 36.8% in biomass. An analysis of the ruffe diet revealed that Gammaridae and Chironomidae were the principal components in all age-classes. The ruffe population was comprised mainly of individuals 3+ and 4+ years of age. The mean total length (TL) of ruffe was 13.39 ± 1.79 cm (minimum-maximum: 6.12-19.93 cm). The growth in length calculated using the von Bertalanffy equation was Lt = 21.3[1-e-a161(t-0,335)]. Ruffe growth rates were found to slow as the fish aged. Ruffe growth variations were linked to population density, fish community structure in the water body, abundance of invertebrates (zooplankton and zoobenthos), and type of water body.