Percent germination and length of hyphae of germinated Glomus mosseae spores, cultivated on water agar, decreased significantly in the presence of Aspergillus niger; this decrease was independent of any change in pH of the medium. Soluble and volatile compounds produced by A. niger significantly decreased percentage spore germination and the hyphal length of G. mosseae on water agar. The decrease caused by volatile compounds was significantly greater when A. niger was grown on malt extract agar. Shoot dry weights of maize and lettuce plants cultivated in soil in pots, and percentage arbuscular mycorrhizal (AM) root colonization of plants grown either in sand: vermiculite tubes inoculated with G. mosseae spores or in soil in pots with soil inoculum, were unaffected by A. niger when this saprobe was inoculated 2 wk after G. mosseae. Shoot dry weights and percentage AM colonization of plants decreased when the saprobic fungus was inoculated at the same time or 2 wk before G. mosseae. However, the metabolic activity resulting from AM colonization, measured as the percentage of mycelium showing succinate dehydrogenase activity, decreased in all treatments. The population of A. niger decreased when inoculated to the rhizosphere of plants at the same time as, or 2 wk after, G. mosseae, but not when it was inoculated 2 wk before G. mosseae. Our results show that G. mosseae decreases the saprobic fungal population through its effect on the plant, whereas A. niger, by the production of soluble or volatile substances, inhibits G. mosseae in its extramatrical stage.