Length–weight relationships (LWR) were determined for three fish species captured in the Mangueira Lake, State of Rio Grande do Sul (RS/Brazil). Specimens were caught in two zones of the lake (north and south) using gill nets (5–70 mm mesh size) at two occasions: May and August 2006. The specimens of Odontesthes humensis, Odontesthes perugiae and Cyanocharax alburnus were measured for total length (TL, to the nearest 0.1 cm) and weight (to the nearest 0.01 g). Previously unavailable in FishBase, maximum total lengths presented for Odontesthes humensis and Odontesthes perugiae species in this study are new records. Was observed that the three species were following allometric growth b higher than three. The LWR data will be useful for deriving future sustainable management and conservation strategies.