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The capacity of three active Mn(IV)‐reducing isolates to dissolve Mn in sterilized samples of two Egyptian soils and a pure sand enriched with MnO2 were studied. These isolates were identified as Penicillium variable (P. v.), Aspergillus niger (A. n.) and Streptomyces exfoliatus (S. e.). The data indicated that inoculation with the fungi and actinomycete mentioned increased the soil contents of water soluble + exchangeable manganese (Mnws+ex) but decreased the easily reducible form (Mner). The increase in Mn‐mobility depended on soil type, organism used and time of incubation. The maximum level of Mnws+ex appeared after 14 days in the 3 soil samples. The release of Mn (II) ranged from 19.6 to 49.4 ppm in the sand samples, from 34.8 to 53.3 ppm in samples of a clay loam soil and from 9.9 to 19.8 ppm in samples of a calcareous sandy loam soil. The increase in Mnws+ex was at the expense of Mner but not in stochiometric amounts. The organisms tested can be ranked according to their capacity to reduce MnO2 in the following order (for all soils) Streptomyces exfoliatus > Aspergillus niger > Penicillium variable. Statistical analysis of the data revealed significant differences due to inoculation, soil type, incubation time and their interactions.

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