Ultrastructural studies were conducted on poplar leaves from the field naturally infected with rust (Melampsora larici-populina.Melampsora medusae) exhibiting chlorotic flecking, necrotic spotting, necrotic and healthy uredinia. Elements of resistance or host defence mechanisms, namely collars, wall outgrowths, encasements, condensation and vacuolation of cytoplasm, disappearance and breakdown of organelles, and cell necrosis were exhibited by both resistant and susceptible cultivars, the difference being the extent to which each factor was expressed. Individual infections occasionally elicited different responses within the same host, highlighting inoculum diversity. The pathogenesis of M. larici-populina and M. medusae to their respective hosts was essentially similar.