Abstract: A glass house experiment was chosen to study the influence of Alternaria alternata and Rhodotorula mucilaginosa on the activities and functions of mycorrhizal fungus Glomus mosseae during growth of Phaseolus vulgaris. This investigation revealed that mycorrhizal plants exhibited improvements in growth compared with non-mycorrhizal ones. In addition, the paired inoculation of G. mosseae with R. mucilaginosa improved plant growth compared with either single inoculation with G. mosseae or paired with A. alternata. Phosphatases activity, dry weight, chlorophyll, carbohydrates and protein contents of mycorrhizal plants were significantly increased in the presence of R. mucilaginosa. It was also noticed that R. mucilaginosa exhibited higher mycorrhizal colonization including; higher percentage in mycorrhizal infection (F%), more colonization intensity (M%) and higher arbuscular formation (A%) compared with single inoculation of AM fungus as well as in case of A. alternata all over the experimental periods. Mycorrhizal plants dependency was increased all over the experimental periods in the presence of A .alternata while, it was decreased in the presence of R. mucilaginosa during the current work. It could be concluded that the benefits of symbiosis of AM fungus and Phaseolus plants was increased in the presence of R. mucilaginosa, while it was decreased in the presence of A. alternata. Key words: Arbuscular mycorrhiza, Glomus mosseae, A. alternata. R. mucilaginosa, soil fungi, symbiosis.