Summary.This work is the first report on length–weight relationships for 15 fish species from the Lower Uruguay freshwater ecoregion. Fishes were collected between 2008 and 2010 in eight streams (Cañada del Sauce, San Luis, Don Esteban Chico and Grande, de la Palmita, Lencina, del Sauce and del Totoral), all tributaries to the Negro River (Uruguay).A standardized fishing effort (50 electric pulses along 100 m) with an electrofishing device (Type IG600T, Hans Grassl GmbH, Schönau am Königssee, Germany) was conducted in each wadeable stream reach. New maximal standard lengths and total weight are given for five and 13 fish species, respectively.