An indigenous bacterial strain capable of utilizing p-nitrophenol (PNP) as the sole carbon, nitrogen and energy source was isolated from a river sediment in Buenos Aires (Argentina). The 16S rDNA sequence showed a close relationship to Rhodococcus wratislaviensis. This indigenous bacterial strain degrades 0.36 and 0.72 mM PNP in 34 and 56 h, respectively, and releases the nitro group from the compound as nitrite. Aerobic biodegradation assays were performed using a 2-L microfermentor at 28 degreesC with agitation (200 rpm). Biodegradation was evaluated by spectrophotometry, gas chromatography and microbial growth. As the nitrite released during PNP degradation is also an environmental toxic agent it was removed by adding a denitrification step in an anoxic process. Detoxification was evaluated by using a Daphnia magna toxicity test. Toxicity was not detected after the combination of both, an aerobic biodegradation test and denitrification in an anoxic process. (C) 2004 Elsevier Ltd. All rights reserved.552103108