Aspergillus niger, a fungus capable of citric and oxalic acid production, was grown on solid medium amended with selected insoluble metal compounds (ZnO, Zn3(PO4)2 and Co3(PO4)2). While A. niger was able to grow on and solubilize all three metal compounds, a crystalline precipitate was observed to form eventually in the clear zone of solubilization under and around the colony. These crystals were extracted from the agar, examined by SEM and X-ray micro-analysis and identified as metal oxalates by performance hplc and gas chromatography–mass spectrometry. The ability of A. niger to precipitate other metal oxalates was investigated with cadmium, copper and manganese phosphates and strontium nitrate. Metal oxalates were again produced and it was found that the different metals resulted in different and characteristic crystal morphologies. A. niger is therefore capable of transforming inorganic insoluble metal compounds into organic insoluble metal compounds. The significance of this process as a means of toxic metal immobilization is discussed.