Abstract This study aimed to isolate and characterize a microbial culture able to degrade sulfonamides. Sulfamethoxazole (SMX)-degrading microorganisms were enriched from activated sludge and wastewater. The resultant mixed culture was composed of four bacterial strains, out of which only Achromobacter denitrificans PR1 could degrade SMX. This sulfonamide was used as sole source of carbon, nitrogen and energy with stoichiometric accumulation of 3-amino-5-methylisoxazole. Strain PR1 was able to remove SMX at a rate of 73.6±9.6μmolSMX/gcelldryweighth. This rate more than doubled when a supplement of amino acids or the other members of the mixed culture were added. Besides SMX, strain PR1 was able to degrade other sulfonamides with anti-microbial activity. Other environmental Achromobacter spp. could not degrade SMX, suggesting that this property is not broadly distributed in members of this genus. Further studies are needed to shed additional light on the genetics and enzymology of this process.