A screening of microorganisms with para-site-specific hydroxylation activity of aromatic compounds has been carried out by a three-step screening procedure involving a newly established micro-plate assay method. About 1300 strains isolated from about 5000 soils showed the activity, The hydroxylation of aromatic compounds by one of the isolates, designated Mycobacterium sp. strain 12523, which had the highest activity, was examined in detail. It produced p-phenols with high selectivity without o- or m-site hydroxylation. The yield of hydroquinone from phenol by strain 12523 was 97 mol%.