The dependence of maximum, minimum and mean target strength (TS), in both horizontal and vertical planes, on the body length, weight and swimbladder volume of European freshwater fish species (trout-Sahno trutta, perch-Perca fluviatilis, bream-Abramis brama, roach-Rutilus rutilus, carp-Cyprinus carpio and bleak-Alburnus alburnus) was studied. TS was measured with a split-beam echosounder operating at 120 kHz. The orientation of the fish towards the transducer was found to be the most important parameter that affected the TS. Body length and fish species were less important and variations in swimbladder volume contributed little. Regression models for TS, length, weight and swimbladder volume were fitted to the data for both individual fish species and pooled for all species. The side aspect in the lateral plane gave the strongest echo for all fish species except trout. Here, the dorso-ventral aspect gave the most intensive echoes. (c) 2005 Elsevier B.V. All rights reserved.