A polychlorophenol-degrading bacterium, Sphingomonas sp. strain P5, was grown in 2,6-dichlorophenol(26-DCP)-limited. 2,3,6-trichlorophenol(236-TCP)-limited, 2,4,6-trichlorophenol(236-TCP)-limited, 2,3,4,6-tetrachlorophenol(2346-TeCP)-limited, and pentachlorophenol(PCP)-limited chemostat cultures at a dilution rate of 0.02 +/- 0.002 h(-1). The cultures were analyzed for the yield coefficient for growth on chlorophenol during steady-state conditions. The average growth yields coefficients (as carbon conversion efficiencies) were 0.252, 0.230, 0.219, 0.157, and 0.121 mol C mol C-1 for 26-DCP, 236-TCP, 246-TCP, 2346-TeCP, and PCP respectively. The differences in growth yield can be interpreted in terms of the energetics of chlorinated carbon metabolism; i.e. substitution of the phenol moiety reduces the available metabolic energy by one electron per chlorine. The growth yield coefficients on chlorinated phenols were lower than the yield coefficients of heterotrophic growth reported in the literature on non-chlorinated and aliphatic compounds. Metabolic origins for low growth yield coefficients on (chlorinated) aromatic compounds are postulated.