Abstract: The paper presents a preliminary discussion of the results of the first Polish attempt to estimate the in situ target strength TS (dB). The subject of the study was the freshwater fish vendace Coregonus albula L. This species is a typical planktivore which inhabits the pelagic zone of Lake Pluszne (903 ha area, 51 m maximum depth) located in northeastern Poland. A Simrad EY-500 type split beam echosounder at a frequency of 120kHz and with a beam width of 7x7 degrees and pulse duration of 0.3 ms was used for acoustic measurements. The trace tracking method with an EP 500 program were used to analyse the data. Control fishing in various water layers was conducted made using pelagic trawl. The dependencies of TS (dB) on fish body length L (cm) were determined for the average values of TS_S.A. = 20 log L -66.8 ± 0.3 SE, TS_{S.A. traces} = 20 log L - 66.4 ± 0.2 SE and TS_max = 20 log L -65. 7 ± 0.3 SE.