Sequential degradation of phenol and cyanide was achieved by a commensal interaction between phenol-degrading and cyanide-degrading bacteria. Phenol-degrading bacteria degraded the phenol rapidly to nontoxic compounds which cyanide-degrading bacteria could successively metabolize as a carbon source for growth. By UV spectroscopy, the phenol-degrading bacteria were presumed to act via a meta-cleavage pathway to yield an intermediate which was metabolized as a carbon source by cyanide-degrading bacteria. By this commensal interaction, 500 mg dm(-3) of phenol and 26 mg dm(-3) of cyanide were completely degraded within 40 h in the batch culture. Using a two-stage reactor system, continuous degradation of phenol and cyanide was also realized.