Microorganisms of bovine rumen fluid have been studied as biocatalysts for the reduction of different N-oxide containing aromatic heterocycles, i.e. quinoxaline N1,N4-dioxide, phenazine N5,N10-dioxide, indazole N1-oxide, benzofuroxan and furoxan. In anaerobiosis, the microorganisms biocatalyzed the reduction of some N-oxides, quinoxaline and phenazine dioxides, to yield the corresponding heterocycle while in the case of benzofuroxans the heterocycle-opening, o-nitroaniline was generated as the unique product. Contrarily, the furoxan was not biotransformed in the studied conditions. The indazole N1-oxide was biotransformed very slowly and incompletely in the corresponding indazole. Only one of the studied indazole N1-oxide derivative was completely converted to a new compound, different to the expected reduced indazole. For that, a semi-preparative scale biotransformation was performed and the new product was spectroscopically identified.