Abstract: The target strength of freshwater fishes was measured in cages using Biosonics 101 dual-beam echosounder. The TS-length and TS-weight relationships were determined for the mixed population of the Dobczyce reservoir and separately for roach, perch, ruffee, bream and pike. The results for small and larger fish suggest that TS for small fish decreases much faster than could be expected from the general 20LogL relationship.