Individual males in many animal species exhibit discrete modes of behaviour(1-3), but the genetic mechanisms underlying these differences are poorly understood. Here we investigate the genetics of the isopod crustacean Paracerceis sculpta, in which three different types of males coexist, each distinguishable from the others by their behavioural and morphological phenotypes(4,5). Within families, alleles of the gene encoding the enzyme phosphoglucomutase (Pgm gene) are associated with particular male phenotypes, although no significant association between these characters exists population-wide. This suggests that Pgm is closely linked to a single genetic locus which controls male phenotype. We call this the alternative mating strategy (Ams) locus. We present evidence that two other factors-an autosomal gene, transformer(Tfr), and an extrachromosomal factor-interact with primary sex determination loci and with alleles at Ams, causing certain individuals to change sex, thereby biasing family sex ratios. A model based on our genetic analysis suggests that : first, polymorphism in male behaviour is controlled by the mendelian segregation of three alleles at the Ams locus; second, that family sex ratio is influenced by alternative alleles at the TFr locus whose expression is influenced by the extrachromosomal factor; and third, that TFr and Ams interact epistatically to determine the sex of the individual and, if male, its behaviour and external morphology.