Chromosome numbers of 37 species of Casuarina are reported, and found to conform with a suggested division of the genus into major species groups. The extra-Australian species groups are probably ancient, and are at the primary diploid level of the genus (x = 8 or 9). Somatic chromosome numbers ranging from 28 to 20 occur in the Australian stocks, and chromosome evolution has possibly involved an ancient polyploidy, followed by stepwise reduction in number. Later evolution in the distyla species group has involved no changes in the haploid chromosome number (n = ll ) , but polyploidy within the species has been frequent.