Under normal conditions sex is considered to be strongly determined by genotype. Nevertheless, influence of environmental factors on sex differentiation in fish has long been suggested due to intersexual features detected in the gonads of some gonochoristic individuals and by the presence of skewed sex ratios in wild populations. Recent data on both reptiles and fish suggest that temperature could be mainly affecting sex differentiation rather than sex determination. Many fish species are characterised by a sexual growth dimorphism, which makes monosex rearing most advantageous for aquaculturists. In future, influence of environmental factor like temperature, can be used for sex differentiation in aquaculture.