SUMMARYThe host-parasite relationship betweenFusariumsp. andStephanoma phaeospora, a biotrophic contact mycoparasite, was studied using both light and electron microscopy. The mode of parasitism as well as features of the host-parasite interface were determined to be similar to those previously reported for theAlternaria tenuis-Gonatobotrys simplexinteraction. Plasmodesmata, 24 to 28 nm in diam, traversed the interface between the two fungi. Virus-like particles, 40 to 47 nm in diam, were observed only in nuclei ofS. phaeospora.