Using light and electron microscopy, mycoparasitism byPiptocephalis unisporawas investigated. Parasitism byP. unisporais restricted to certain members of the Mucorales.Piptocephalis unisporais also shown to infect germinating spores and hyphae of the potential thamnidiaceous host,Phascolomyces articulosus. Appressoria, penetration pegs, and haustoria are formed during the infection process but further development of the mycoparasite ceases. Reaction papillae and material present at infection sites and around degenerate haustoria are associated with the mechanism of resistance. The process of infection is compared with previous studies of the infection ofCokeromyces recurvatus(Thamnidiaceae) byPiptocephalis unispora.