The effect of phenol concentration on growth and biodegradative capacity of Ralstonia eutropha regarding phenol was examined. Kinetic analysis indicated that phenol had a strong inhibitory effect on phenol metabolism and growth rate, although biomass yields remained constant, indicating that this phenomena was not caused by increased maintenance requirements. Measurements of specific enzyme activities involved specifically in the catabolic pathway of meta fission of phenol indicated that gene expression cannot explain the diminished metabolic rates at inhibitory phenol concentrations. This phenomenon is due to in vivo inhibition of enzyme activities and notably to phenol hydroxylase activity. Furthermore, other nonmetabolizable organic alcohols provoked a similar effect on both specific growth rate and phenol hydroxylase activity, indicating that inhibition was probably associated with modified membrane fluidity, partially offset by a change in the fatty acid composition of cellular lipids.