The fine structure of the haustorium of the pycnio-aecial stage ofMelampsora pinitorquain infected cells of cotyledons ofPinus pineawas studied. Some features were found to be different from those of the uredial-telial stage of the same rust onPopulus tremula.The mother cell wall lacked thichening at the penetration site; the penetration site was found to be very wide; the mother cell wall was continuous with the haustorial wall and the fungal wall was clearly distinct from the host wall; the haustorium had a hypha-like shape and a septum in its proximal region. The interface between the haustorium and the host cell, and the relationship of some host organelles with the haustorium appeared similar to those of the uredial-telial stage ofMelampsora pinitorquaon aspen.