ALTERNATIVE male mating tactics are widespread among animal taxa(1-3), but there are few well documented examples of genetic polymorphisms for them(4-6). The dimorphism in male courtship behaviour between independent and satellite ruffs, Philomachus pugnax(7,8) (a lekking sandpiper), has often been cited as a potential example but this has been questioned(9-10) because of the lack of data(11) and the widespread phenotypic plasticity in the development or expression of alternative tactics in other species(1-3,9,12-14). By rearing ruffs in captivity, we now show that differential morph development is genetically controlled and consistent with a single-locus, two-allele autosomal genetic polymorphism. Several potentially relevant environmental factors do not appear to alter behavioural development.