The inoculum potential for arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungi were investigated in thinned and uncut control stands in a northern Arizona ponderosa pine forest. A corn bioassay was used to determine the relative amount of infective propagules of AM fungi, and a ponderosa pine (Pinus ponderosa) bioassay was used to determine the relative amount of infective propagules of EM fungi. Three stands of each treatment were sampled by collecting soil cores along 10 randomly chosen transects within each stand. The relative amount of infective propagules of AM fungi was significantly higher in samples collected from the thinned stands than controls. Conversely, there was a slight decrease in the relative amount of infective propagules of EM fungi in samples collected from thinned stands in comparison to the controls; however, this difference was not significant. These preliminary results indicate that population densities of AM fungi can rapidly increase following restoration thinning in northern Arizona ponderosa pine forests. This may have important implications for restoring the herbaceous understory of these forests because most understory plants depend upon AM associations for normal growth.