Abstract: In 2005, the distribution, density, and structure of pelagic fish resources was studied in lakes Mielno (363 ha) and Maróz (332 ha) located in the Łyna catchment basin through which the Marózka River flows. The studies were conducted with a Simrad EY-500 echosounder (120 kHz, 7x7 deg, 0.3 ms, split beam version) and a pelagic trawl for monitoring catches. At the outset of the 1990s, vendace dominated the commercial catches of both lakes reaching about fifteen kg·ha^-1. Significant changes have occurred in the structure of the pelagic ichthyofauna in recent years in the studied lakes; the disappearance of vendace and the dominance of bleak and roach in the catches indicates that the tempo of eutrophication has increased.