A Mn-depositing fungus, Acremonium-like hyphomycete strain KR21-2, was isolated from a Mn deposit occurring on the wall of a storage bottle containing Mn(III, IV) oxide-coated streambed pebbles and stream water. 18S rRNA gene sequence analysis revealed that strain KR21-2 was phylogenetically related to members of the order Hypocreales within the class Ascomycetes. The spent culture medium at the stationary phase of fungal growth contained a 54-kDa protein capable of depositing Mn oxides. The enzymatic activity was inhibited by azide and o-phenanthroline. The Mn(II)-oxidizing protein possessed a laccase activity, as indicated by direct oxidation of p-phenylenediamine and 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid). These results are consistent with the role assumed for laccase-like multicopper oxidase, which is proposed to be involved in the Mn(II)-oxidizing factors from some bacteria. Unlike laccases of basidiomycete fungi, however, the protein of strain KR21-2 did not produce soluble Mn(III) species in the presence of either of the Mn chelators pyrophosphate and malonate. This is the first report on the possible involvement of laccase and/or multicopper oxidase in Mn oxide deposition by ascomycetes (including their anamorphs) ubiquitous in natural environments.