Hyphae and haustoria of the pycnio-aecial stage ofCronartium flacciduminPinus pineaandP. pinasterwere observed with a TEM. This characteristically systemic and long-lasting monokaryotic mycelium was examined in primary needles of young seedlings with the first symptoms of infection, in the stem of the same seedlings just invaded by the rust and in cortical tissues of infected internodes of older plants. The described ultrastructural features, generally referable to all examined materials, regarded: the wall, capsular sheath and contents of the intercellular hyphae; the haustorial mother cell and the penetration peg; the general morphology, wall, extrahaustorial matrix, collar and encasement of the haustorium; the close proximity to the extrahaustorial membrane of the host cell endoplasmic reticulum and Golgi apparatus. Some aspects ofC. flaccidummonokaryotic haustorium and related structures are discussed and compared to those reported for monokaryotic and dikaryotic haustoria of other rusts.