Twenty-one allozyme loci in samples of wild-caught and hatchery-reared Indian major carps from Bangladesh were analysed. Bayesian model-based clustering analysis revealed the presence of four taxa, corresponding to the three known species along with a fourth unknown taxon present in two hatchery samples. Individual admixture coefficients showed that 24% of all hatchery-reared fishes were hybrids, whereas a single hybrid was observed in the wild-caught samples. Only catla Catla catla x rohu Labeo rohita and mrigal Cirrhinus cirrhosus x rohu hybrids were observed, the vast majority of which were F, hybrids, though five individuals represented putative backcrosses. Mitochondrial DNA analysis revealed that catla x rohu hybridization primarily involved catla males and rohu females, whereas mrigal x rohu hybrids primarily resulted from rohu males and mrigal females. Despite the high percentage of F-1-hybrids in hatchery samples, reproductive barriers among species have so far precluded widespread introgression. Continued hybridization may eventually lead to a breakdown of species barriers, thereby compromising the genetic integrity of the species in the wild, and leading to production losses in aquaculture. (c) 2005 The Fisheries Society of the British Isles.