We have discovered a cytoplasmically inherited infectious agent that inhibits meiosis in a species of Coprinus, a basidiomycetous fungus. From infectivity, filtration, centrifugation and ultrastructural studies we believe the agent to be a mycoplasma. The agent is highly infectious to several strains of the host species and is capable of spreading rapidly through infected hosts. No pathological effect has been seen on any aspect of growth or differentiation of the fungus except for the inability of infected strains or undergo meiosis. The failure of meiosis results in mushrooms that do not produce the normal black spores and are therefore pale in colour. The paleness represents a simple assay for the presence and activity of the infectious agent. Infected hosts do not display any ultrastructural abnormalities in the vegetative stages, only in the cells in which meiosis should occur. In the meiotic cells, at the time when normal cells are undergoing synapsis and synaptinemal complexes are forming, the vacuoles of the infected cells become occupied with vesicular, membrane-bound bodies resembling in shape and form mycoplasmas. Extracts from infected clones may be filtered through 0.2-mum filters and retain full infectivity. The infectious material may be pelleted from such extracts at only 10 000g. Migration experiments, as well as the filtration studies, rule out involvement directly of nuclei. The high rate of infection and spread of the mycoplasma through the host, combined with the anatomical simplicity of the host, make this an ideal system in which to study the basis of infection. The singularity of the pathological effect make this host-parasite association useful in studying both the underlying mechanisms of mycoplasma pathogenicity and to investigate the regulation of meiosis. This is only the second report of mycoplasmas in fungi.