Phenol degradation by Candida tropicalis and its fusant, which is produced using protoplast fusion as a selective technique, is evaluated under batch and high concentration conditions. The respirometric data show that oxygen uptake activities of both yeast strains peak at pH 7.0 and 32°C, but the fusant is more active than the control strain. Although the data show that both yeast strains are capable of sustaining discernible degradation in the presence of phenol inhibition, however, the C. tropicalis fusant is capable of attaining better phenol degradation than the control strain and it is less susceptible to phenol inhibition. Under the conditions tested, C. tropicalis is completely inhibited at phenol concentrations ⩾3,300 mg/L, whereas for the C. tropicalis fusant complete inhibition is absent until phenol concentrations are ⩾4,000 mg/L. The observed cell yields of both yeast strains are virtually identical and remain fairly constant at approximately 0.5 mg MLVSS/mg C6H5OH (MLVSS: mixed liquor volatile suspended solids). © 1998 John Wiley & Sons, Inc. Biotechnol Bioeng 60: 391–395, 1998.