The 13 somatic chromosomes of a male marsupial rat-kangaroo (Potorous tridactylis apicalis) are described and measurements are given. Each of the five pairs of autosomes and the three unpaired sex chromosomes are morphologically distinct. During pachytene there are five autosomal bivalents and one sex-chromosome trivalent. In early subcultutes of muscle fibroblasts a large proportion of the cells were polyploid and there were a number of cells with extra and/or missing Y chromosomes. Because of the simplicity of its karyotype, this mammal should prove extremely useful in studies of chromosome evolution in cultured cells and experimental studies involving changes in chromosome number and structure.