The ability of the soil fungi Aspergillus niger and Serpula himantioides to tolerate and solubilize manganese oxides, including a fungal-produced manganese oxide and birnessite, was investigated. Aspergillus niger and S. himantioides were capable of solubilizing all the insoluble oxides when incorporated into solid medium: MnO2 and Mn2O3, mycogenic manganese oxide (MnOx) and birnessite [(Na0.3Ca0.1K0.1)(Mn4+,Mn3+)2O(4)center dot 1.5H2O]. Manganese oxides were of low toxicity and A. niger and S. himantioides were able to grow on 0.5% (w/v) of all the test compounds, with accompanying acidification of the media. Precipitation of insoluble manganese and calcium oxalate occurred under colonies growing on agar amended with all the test manganese oxides after growth of A. niger and S. himantioides at 25 degrees C. The formation of manganese oxalate trihydrate was detected after growth of S. himantioides with birnessite which subsequently was transformed to manganese oxalate dihydrate. Our results represent a novel addition to our knowledge of the biogeochemical cycle of manganese, and the roles of fungi in effecting transformations of insoluble metal-containing compounds in the environment.