Almost all mammals (including humans) share the same sex chromosomes, whereby individuals with a Y chromosome are male (XY) and individuals with two X chromosomes (XX) are female. Male elephants, cats and men all have a very similar Y chromosome that evolved in their common ancestor 180 million years ago. In fishes, sex determination is not as stable. Many different genes can determine if an individual turns into a male or a female and in some species sex is not even genetically determined. Here, Anna Feller and I studied the sex determination in three crosses of East African cichlid fishes, two species pairs from Lake Victoria and one from Lake Malawi. We found a different sex determiner in each cross, even though the Lake Victoria species split less than 15,000 years ago. One of the Lake Victoria sex determiners is found on the same chromosome as the sex determiner in the Malawi cross, but in the Victoria cross, we found a dominant female determiner (W ) and in the Malawi cross, a dominant male determiner (Y). So despite reuse of the same chromosome, the sex determination system is likely different between these species. Our study highlights how extremely labile sex determination is in haplochromine cichlid fishes and that many different genes can act as sex determiners. Rapid turnover in sex determiners may also contribute to their exceptionally rapid diversification.