The ECOLOG dual‐beam echo‐counting and sizing system is used in both a laboratory and a field situation with moderate densities of demersal fish. Free swimming fish in a large tank give estimates of backscattering cross section (σbs) with low variance. Application in field surveys confirms that dual‐beam acoustic data provide information on both size composition and density distribution, which is different than single‐beam estimates but similar to and at least as precise as that provided by trawl nets.