Resumo. The City Biological Reserve of 'Serra dos Toledos' is an important remaining tropical rain forest in south Minas Gerais statewhich has high biodiversity and water resources. Studies related to soil quality are needed in the reserve management plans, which aim to maintain or to improve the environmental sustainability of these areas. Thus, the purpose of this work was to evaluate the inoculum potential of arbuscular mycorrhizal fungi (AM) in samples of typical soil of that reserve. The quantifcation of propagules of this microbial group that establishes mutualistic symbiosis with most plant species, it is important for assessing the soil quality of the reserve. Three areas inside and one outside the Reserve were set to study, where soil samples were removed at depth 0 to 10 cm, in two seasons, winter 2008 and summer 2009. The samples were sent to the Laboratory of Microbiology of the University of ¿Itajubá¿ for quantification of the following microbiological attributes related to AM fungi: lengths of total and active extra-radical mycelium by the method of induced fluorescence with fluorescein diacetate, density and phenotypic diversity (morphotypes) of spores, percentage and intensity of root colonization. The average results were compared by Duncan 5% and subjected to multivariate analysis. The results showed that the potential for MA fungal inoculum had a higher effect of areas in relation to the studied periods, with greater length of active and total extra-radical mycelium, a higher proportion of active mycelium in relation to the total and a greater diversity of spores in soil from the City Biological Reserve of ¿Serra dos Toledos¿, compared to the outside soil under pasture. The inoculum potential is not directly related to the soil fertility, where that outside the reserve, pasture, despite the best fertility were found smaller values of mycelium and diversity of MA fungal spores.