The metabolism of chloroguaiacols by a soil bacterium was studied. The strain was isolated by enrichment, with guaiacol as the sole carbon and energy source, and identified as a Rhodococcus ruber CA16. None of seven chlorinated guaiacols supported bacterial growth. However, ultraviolet spectroscopy, chloride release, and oxygen consumption showed that resting cells grown on guaiacol degraded completely 4-chloroguaiacol, 5-chloroguaiacol, and 6-chloroguaiacol and, to a lesser extent, 4,5-dichloroguaiacol. Gas chromatographic analysis suggested microbial formation of 4-chlorocatechol and 4,5-dichlorocatechol from 4-chloroguaiacol and 4,5-dichloroguaiacol, respectively. Although mono- and dichloroguaiacols did not affect the strain's ability to grow on guaiacol, chlorocatechols completely arrested growth. The role of chlorocatechols in chloroguaiacol metabolism by this guaiacol-degrading bacterial strain is discussed.