Melampsora rust is a serious foliar disease of poplar in France and resistance to this pathogen is one of the most sought after traits in poplar breeding programs. However, relatively little is known concerning the physiological and molecular aspects of poplar-rust interactions. Our goal is to study this interaction so as to identify genes involved in rust resistance that may then be used as markers by poplar breeders, as well as deepening our biological understanding of this interaction. In order to investigate this host-pathogen relationship we have developed a model system consisting of leaf disks of a Populus nigra x P. deltoides clone inoculated with two different Melampsora larici-populina races, resulting in either a compatible or an incompatible reaction. Histological studies of rust development after inoculation have allowed us to determine at what point the two reactions start to differ. The incompatible response of the host plant to the pathogen infection seems to be rapid and highly localised to a limited number of cells while most of the leaf remains healthy. Presently, we are studying the molecular aspects of poplar-rust interaction by Northern analysis and we are using different sequences coding for genes known to be involved in defence reaction as probes. Due to the very localised response of the host to pathogen attack, we are also attempting to characterise the spatiotemporal expression pattern of such defense genes in poplar leaves through in situ hybridization. The first results obtained with this technique will be presented.