There have been few reported family studies of gene segregation in marsupials and no report on genetic linkage in a marsupial species. The paucity of such genetic data probably stems from the lack of marsupial species sufficiently well adapted to a laboratory environment to give the abundant progeny from controlled crosses which are required in such work. A colony of the fat-tailed insectivore Sminthopsis crassicaudata (Marsupialia: Dasyuridae), which was established in the R. A. Fisher Laboratories, University of Adelaide, more than 20 years ago, has now provided unusual linkage data. These data indicate that the linkage situation in this marsupial species differs markedly from that in eutherian mammals. Extremely large differences exist between the sexes in the values of the recombination frequencies, with much closer linkage occurring in females. Cytological examination of meiosis in males and females has revealed a major difference between the sexes in chromosome behaviour.