The length–weight relationships (LWRs) were studied for 814 individuals covering eight families and 12 species of fish, e.g., Botia dario, Botia lohachata, Channa orientalis, Esomus danricus, Heteropneustes fossilis, Macrognathus pancalus, Nandus nandus, Ompok pabda, Pethia phutunio, Pethia ticto, Trichogaster fasciata and Trichogaster lalius captured from the Gajner beel floodplain, northwestern (NW) Bangladesh. Samples were collected occasionally from catches of fishermen from November 2014 to October 2015 using different types of traditional fishing gear, including gill net (mesh size ranges from 1.0 to 3.0 cm), cast net (mesh size ranges from 1.0 to 2.0 cm), square lift net (mesh size ~1.0 cm) and conical and box trap. The total length (TL) and body weight (BW) were measured for each individual. The allometric co-efficient b values ranged between 2.64 (B. lohachata) to 3.27 (N. nandus) and were highly significant, with p < .001 for all species. Additionally, for the first time, the study provides LWR information for B. dario, B. lohachata, E. danricus, O. pabda, T. lalius and P. phutunio, beneficial for fishery biologists and to update the online database (FishBase). The study also recorded a maximum length for P. phutunio. The present observations are significant for the sustainable management and conservation of these fishes in the Gajner beel floodplain, NW Bangladesh and the surrounding ecosystems.