Activity of ammonia-oxidizing bacteria (AOB) to simultaneous variation in Zn2+ concentration (0.01-3.5 mg/L), temperature (23-33 degrees C), and AOB concentration (3-30 x 10(6) gene copies/mL) in a steel industry wastewater treatment plant was evaluated. Two equations were developed to describe the lag period (i.e., AOB acclimation) and ammonia oxidation rate (i.e., growth of the AOB) depending on the variables. AOB concentration and temperature both had significant effects on lag period and the ammonia oxidation rate. Zn2+ concentration only had a significant effect on ammonia oxidation rate at 5% alpha-level. There was a significant interaction between AOB concentration and temperature for both lag period and ammonia oxidation rate. The effects of the variables were not significant when AOB concentration was higher than 2.0 x 10(7) copies/ml. There was no visible shift or changes in AOB communities based on DGGE analysis with amoA gene primers. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.