Abstract: Theoretical investigation of sound extinction cross section of zooplankton individual is made on the basis of optical theorem. To find general properties of this characteristic, analytical model, describing the scattering at krill individual. is used. The extinction cross section is evaluated for various krill species. The dependence of the extinction cross section on the sound frequency is obtained. The paper presents the influence of the acoustical properties of the biological tissue and geometrical form of individual on the extinction cross section. The role of sound absorption in krill tissue is discussed.