Melampsora leaf rust is the most widespread and serious foliar disease for poplar species and their hybrids. Severe damage has been attributed to these pathogens including growth reduction the following year or even death of the plant (1). Our objective is to characterize histological and biochemical level the response of poplar to rust infection. Toward this end, we use an in vitro test consisting of leaf disks of Populus nigra x P. deltoides clones inoculated with different physiological races of Melampsora larici-populina. Depending on the poplar clone or on the physiological race used, the response will be compatible or incompatible. In compatible interaction, the first infection symptoms appear 8 to 9 days after inoculation. Sampling are performed daily during the week following inoculation. Pathogenesis development of the fungus was studied by light microscopy and scanning electron microscopy. We describe the early stages of infection for both interactions (compatible, incompatible): spore germination on leaf epidermis, penetration of the developing hyphae through the stomata. A lot of studies have outlined the role of phenolic compounds in defense response (phytoalexines). In order to establish correlations between phenolic content and biological response of the poplar leaf to pathogen attack, we performed kinetic analysis of leaf phenolic compounds after inoculation using high performance liquid chromatography and thin layer chromatography. Biochemical data will be discussed in comparison with histological observations.