When determining the density krill of concentrations it is necessary to know the acoustic cross-section of back-scattering of krill. The target strength can be determined in situ by measuring the echo signal from one specimen resolved by the hydroacoustic instrument. By calculating the mean level of the directivity diagramme of the antenna and the mean level of the standardized echo signal, it is possible to determine the acoustic cross-section of the back-scattering of krill. By experiments, the authors have obtained the dependence of the acoustic section of back-scattering of krill on its length at a frequency of 120 kHz.