Using field emission scanning electronmicroscopy, hyphal surfaces were examined in six lichens, Lasallia papulosa (Ach.) Llano, Xanthoria elegans (Link.) Th. Fr., Peltigera aphthosa (L.) Willd., Stereocaulon depressum (Frey) M. Lamb, Umbilicaria mammulata (Ach.) Tuck, and U. cylindrica (L.) Duby and in material of the same species associated with pathogenic or saprobic fungi. In samples infected by each of Muellerella pygmaea (Körber) D. Hawks., Marchandiomyces corallinus (Roberge) Diederich & D. Hawks., Lasiospheriopsis stereocaulicola (Lindsay) O. Eriksson & R. Sant, and a Penicillium-like fungus, some hyphae were found that differed from those of the lichen mycobiont in size, septation, surface texture, or inclination to collapse. Anomalous hyphae were uncommon in some diseased lichens, in fact no candidate pathogens were detected in S. depressum infected with Arthonia stereocaulina (Ohlert) R. Sant., although extensive mycelia of L. stereocaulicola were evident in the same lichen. The extent of visible colonization by extraneous fungi was greatest in recently killed U. mammulata.