A wide range of man-made chemicals has the ability to interfere with the endocrine system that controls amongst other functions, sexual development. These are called endocrine disruptors (EDs). Steroidal oestrogens are often implicated in the causation of the widely observed sexual disruption in fish and are of both natural and synthetic origin (e.g. ethinyl-oestradiol, which is the active ingredient of the human contraceptive pill). As well as posing potential risks to humans (there are several studies that attempt to link exposure to EDs with the decline of sperm counts, the increased incidence of testicular and prostate cancers, male congenital reproductive abnormalities and infertility rates), EDs are of particular concern for fish as the aquatic environment is often the most important sink for man-made chemicals and sewage waste. There is clearly a need to develop tools for the detection of such chemicals and fish have been recognised as providing the ideal biological system for this purpose, leading to the development of a number of tests by the OECD, including the fish sexual development test (FSDT) The species that have entered a validation phase for the FSDT to date are the zebrafish and the fathead minnow, both of which are routinely used in aquatic ecotoxicological studies. However, we argue that they are not the ideal test subjects for the FSDT. The main disadvantage of these fish species is the lack of a genetic sex marker, which could unequivocally assign sex, leading to an enormous wastage of experimental fish. This is because the main endpoint employed by the FSDT is sex ratio, which in the case of the stickleback can be assigned genetically using a simple test. In the case of alternative models, sex is assigned by means of gonadal histology, a strategy that presents many drawbacks. Firstly, if the stickleback is used, an acceptable sex ratio for the control groups does not need to be defined waiving the risk of test failure. Secondly, the mode of action of a chemical can be better defined as an oestrogen or an androgen for example because any differences between genetic and histological sex can be attributed to the chemical exposure. Thirdly, but most importantly the ability to assign genetic sex increases the power of the test and can dramatically reduce the number of animals used in scientific procedures for both research and regulatory purposes.