A male calico cat was studied cytogenetically and found to have a 43%/57% XY/XX cellular admixture that is thought to have arisen by chimerism. The percentage of XY cells varied in the tissues examined (testes, blood and kidney). Normally descended testes were found which were composed of sterile and fertile seminiferous tubules. The latter had active spermatogenesis and spermatozoa were present in the epididymis, thus accounting for the possibility of ‘fertile calico males’. No female genital structures were present.