Abstract: The paper presents an algorithm for estimation the fish length distribution from acoustic target strength data. The theory of scattering from a tilted cylinder is used for modelling the fish directivity pattern of swimbladdered fish. The model allows for formulating the dependence of target strength on two main components; the maximum target strength and the fish directivity pattern. As both terms depend on fish length, the inverse technique used when processing is performed on introduced conditional distributions. The effect of fish tilt angle is in this way removed, or deconvolved from the target strength distribution.. The resulting fish maximum target strength distribution is further converted into fish length distribution using equivalent parameters for swimbladder morphology. The method is verified on actual data acquired during the Lofoten 2004 survey on spawning grounds of North East Arctic cod (Gadus morhua).