Sugarcane cultivars are polyploid, aneuploid, interspecific hybrids between the domesticated species Saccharum ofjcinarum and the wild relative S. spontaneum. Cultivar chromosome numbers range from 100 to 130 with -10% contributed by S. spontaneum. We have undertaken a mapping study onthe progeny of a selfed cultivar, R570, to analyze this complex genome structure. A set of 128 restriction fragment length polymorphism probes and one isozymewas used. Four hundred and eight markers were placed onto 96 cosegregation groups, based on linkages in coupling only. These groups could tentatively be assembled into 10 basic linkage groups on thebasis of common probes. Origin of markers was investigated for 61 probes and the isozyme, leading to the identification of 80 S. ofJinamm and 66 S. spontanrum derived markers, respectively. Their distribution in cosegregation groups showed better mapcoveragefor theS.spontaneumthanfor theS.of3cinnrumgenomefractionandoccasionalrecombina- tion between the two genomes. The study of repulsions between markers suggested the prevalence of random pairing between chromosomes, typical of autopolyploids. However, cases of preferential pairing between S. spontaneum chromosomes were also detected. A tentative Saccharum map was constructed by pooling linkage information for each linkage group.