The relative DNA contents of individual chromosomes of Sminthopsis crassicaudata (Marsupialia) have been measured by DNA cytophotometry. Measurements of the relative nuclear DNA contents of man and Sminthopsis have been made and show that the DNA content of the marsupial X chromosome is substantially less than that of the X chromosome of man. The implications of this observation with respect to the argument of the conserved X chromosome in mammals are presented.