The effect of inoculum size on the acclimation period and rate and extent of p-nitrophenol (PNP) degradation at high (1-10 mg/L) and low (26 mu g/L) concentrations for two bacteria was determined in defined media as well as industrial wastewater. Increased inoculum size did not affect the acclimation period of either bacterium at high (1-10 mg/L) PNP concentrations. At low PNP concentrations (26 mu g/L), the two bacteria behaved differently. The acclimation period was shortened and both the rate and extent of mineralization of PNP were enhanced by increasing the Corynebacterium sp. inoculum size from 3 X 10(5) to 3 X 10(6) cells/ml. Addition of phosphate or elimination of predators also reduced the acclimation period. Conversely, increasing the inoculum size from 3 X 10(5) to 5 X 10(6) cells/ml of Pseudomonas putida lengthened the acclimation period and reduced both the rate and extent of mineralization. It is suggested that, in a given environment, the success of an introduced species to enhance the degradation of a chemical depends upon (i) concentration of the chemical, (ii) selection of an appropriate microorganism and (iii) utilization of a suitable inoculum size.