Abstract Dependence of the in situ acoustic target strength (TS) of smelt (Osmerus eperlanus (L.)) on the total fish length (L) was estimated from TS measurements made with a split-beam 120 kHz echo sounder with reference to fish length–frequency distributions in trawl catches. The studies were conducted in the eutrophic and turbid Lake Hiidenvesi in southern Finland. Smelt dominated the catches-in-number with proportions at different sampling stations ranging from 91.8 to 100%. A TS–length equation of the form TS = alog L − b was fitted through the modes in the TS-distributions and in the respective fish length distributions. The estimated model (±parameter standard error) was TS = 23.4 (±2.24)log10 L − 68.7 (±1.82). With a pre-defined slope of 20, the fitted equation was TS = 20 log10 L − 65.9 (±0.23). The estimated day–night difference in TS–length relationship was not significant (t-test, p = 0.37).