Salt wedge is a quasi permanent feature at the Río de la Plata estuary, generating strong vertical density gradients. Zooplankton and ichthyoplankton organisms aggregated below the halocline were insonified utilizing a scientific echosounder operating with a 120 kHz single beam transducer. From acoustic data and CTD measurements, a strong coincidence between the depths of the scattering layer and the halocline was proven. The organisms from the zooplankton and the ichthyoplankton were caught with a Nackthai sampler and classified into four functional groups according to their scattering properties. A multiple regression analysis shows fish larvae as the most significant source of sound scattering at the halocline. Finally, the salt wedge across the estuarine regime is described employing an echochart. The halocline here showed a quasi constant depth at 7-8 m with the salt water intruding 100 km upstream.