This review summarises the wide range of toxicological effects of malachite green (MG), a triarylmethane dye on various fish species and certain mammals. MG is widely used in aquaculture as a parasiticide and in food, health, textile and other industries for one or the other purposes. It controls fungal attacks, protozoan infections and some other diseases caused by helminths on a wide variety of fish and other aquatic organisms. However, the dye has generated much concern regarding its use, due to its reported toxic effects. The toxicity of this dye increases with exposure time, temperature and concentration. It has been reported to cause carcinogenesis, mutagenesis, chromosomal fractures, teratogenecity and respiratory toxicity. Histopathological effects of MG include multi-organ tissue injury. Significant alterations occur in biochemical parameters of blood in MG exposed fish. Residues of MG and its reduced form, leucomalachite green have been reported from serum, liver, kidney, muscles and other tissues as also from eggs and fry. Toxicity occurs in some mammals, including organ damage, mutagenic, carcinogenic and developmental abnormalities. However, despite the large amount of data on its toxic effects, MG is still used as a parasiticide in aquaculture and other industries. It is concluded that the potential of alternative parasiticides, like humic acid, chlorine dioxide and Pyceze, should be explored to replace MG. Until then, MG should be used with extreme care at suitable concentrations and at times when the temperature is low. Removal of residual MG in treatment ponds should also be considered.