The times necessary for unacclimated wastewater to degrade low concentration of mono-chloro-, mono-nitro-, and mono-aminobenzoates and -phenols were studied. The time for the substituted benzoates was not reduced either by preadaptation to benzoate or by addition of benzoate during acclimation. The presence of easily metabolized carbon sources (benzoate, acetate, glucose, peptone) did not shorten the time for wastewater to break down m-chlorobenzoate or p-chlorophenol. Preacclimation is recommended for the efficient degradation of certain recalcitrant wastes.