In the Casuarina distyla species group, which includes 13 closely related species, sexual tetraploid forms have been recorded in nine species and apomictic triploids in four. These forms are probably autopolyploids. The triploid apomiots are presumably diplosporous and parthenogenetic, and in two species are pseudogamous, but not in the other two. It is suggested that a transition from the former to the latter breeding system would follow elimination of the need for pseudogamy. A hypothesis is presented that polyploidy and apomixis have arisen together within the species through establishment in a diploid population of a triploid plant which is diplosporous and parthenogenetic. The genetic system involved probably spread through the group during the evolution of the species, giving them a potential for this kind of development.