A cyto-histological study was carried out onPinus sylvestrisseedlings artificially infected with basidiospores ofMelampsora pinitorquaandMelampsora larici- tremulae, in order to differentiate between the two rust fungi and delineate their taxonomical relationships. The general morphology of the early phases of the majority of infections ofMelampsora larici-tremulaeonP. sylvestrisshow similar characteristics to those ofM. pinitorqua.Moreover, inM. larici-tremulae/P. sylvestrisinteractions, the following were the specific characteristics: abnormal germlings and secondary basidiospore production, «cell wall-like appositions» encasing the fungal structures in the host, necrosis of the infection structures, necrosis of the host tissue colonized by mycelium from epidermal to parenchymal cells. Such reaction types reveal the coexistence of pre-and posthaustorial defence mechanisms, typical, respectively, of a non-host and host resistance. This coexistence which is useful in understanding the evolution of this host-rust interaction, was examined. The described defence mechanisms ofP. sylvestrisvs.M. larici-tremulaecould indicate that the two rust fungi are still closely related entities, however diverging, which evolve from «formae speciales» to «species».