A strain of Klebsiella pneumoniae that used aliphatic nitriles as the sole source of nitrogen was adapted to benzonitrile as the sole source of carbon and nitrogen. Gas chromatographic and mass spectral analyses of culture filtrates indicated that K. pneumoniae metabolized 8.4 mM benzonitrile to 4.0 mM benzoic acid and 2.7 mM ammonia. In addition, butyronitrile was metabolized to butyramide and ammonia. The isolate also degraded mixtures of benzonitrile and aliphatic nitriles. Cell extracts contained nitrile hydratase and amidase activities. The enzyme activities were higher with butyronitrile and butyramide than with benzonitrile and benzamide, and amidase activities were twofold higher than nitrile hydratase activities. K. pneumoniae appears promising for the bioremediation of sites contaminated with aliphatic and aromatic nitriles.