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Polycyclic aromatic hydrocarbons (PAHs) are common ubiquitous pollutants existing in nature with high recalcitrance and toxicity. In this study a bacterium capable of aerobic degradation of high molecular weight PAHs (with special reference to pyrene) was isolated by selective enrichment culture technique from oil refinery effluent sludge. The isolate was characterized as Achromobacter xylooxidans by 16S rRNA gene sequence analysis technique. For the first time it is hereby reported a bacterium capable of effectively degrading pyrene (up to 80%), as evident by reverse phase high performance liquid chromatographic analysis (RP-HPLC). After incubation of Achromobacter xylooxidans in minimal salt medium (MSM) containing pyrene, at concentration of 200 mg/L, as sole source of carbon and energy, there was decrease in pyrene concentration concomitant with increase in bacterial cell protein concentration. RP-HPLC analysis revealed that pyrene was degraded into three metabolites viz. I, II and III. The RP-HPLC eluent fraction were collected from 2.5 to 32.0 min by repeated injection of degraded sample, concentrated and analyzed on gas chromatography mass spectroscopy (GC-MS) for metabolite identification. The fraction shows 1-hydroxypyrene, 1-hydroxy-6-methoxypyrene and 1,6dimethoxypyrene as metabolic product of pyrene degradation, on the basis of their m/ z values. On contrary to the reported PAH degradation with reference to pyrene by different isolates till date; the efficient degradation, as evident by RP-HPLC, by this isolate holds a promising potential for planning of bioremediation strategies of contaminated sites. [ABSTRACT FROM AUTHOR]