Glutathione S-transferases (GST) deactivate many toxins, and are thus largely responsible for the organism’s capacity to adapt to the environment. Enzymes of this family can be found in all organisms, excluding anaerobic bacteria and viruses. These proteins can be regarded as a model of the molecular evolution of biopolymers. Some difficulties arise however in fish GST classification, because information about these ancient water organisms is insufficient. This minireview consolidates data about GST in fish and the relations between their molecular structure and function. A comparative analysis has revealed a significant difference between the biochemical properties of mammalian and fish GST.