Xylohypha emmonsii Padhye, McGinnis et Ajello is described as a new dematiaceous opportunistic pathogen of humans and lower animals. This hyphomycete differs from X. bantiana (Saccardo) McGinnis, Padhye, Borelli et Ajello by its inability to grow at 40 degrees C and by the development of short chains of asymmetrically bent to ellipsoid blastoconidia. In brain tissue of mice, experimentally infected with X. emmonsii, the host response to the invading fungus was similar to that observed in mice infected with X. bantiana. In tissues X. bantiana occurred predominantly as long, septate hyphae, in contrast to X. emmonsii, which was more polymorphic.