The inhibitory effect of ammonia on the growth of the polychlorinated xenobiotic-degrading bacterium Mycobacterium chlorophenolicum was examined. The strain is inhibited by both the ionized and nonionized forms of ammonia. At pH 6.9 50% reduction of the growth rate was observed at 6.8 gl(-1) total ammonium. For 23 experiments performed in shake-flask culture at different pH values and ammonium concentrations a growth model based on the extended Monod kinetic fits the data with a deviation of 5.3%. To overcome growth inhibition in bioreactors a pH-controlled feeding strategy was developed for effective cultivation of M. chlorophenolicum at an ammonium level below 0.3 gl(-1). The ammonium addition was controlled on-line by the stoichiometric interdependence of ammonium consumption and pH decline. With this on-line control strategy a biomass concentration as high as 26.2 gl(-1) can be achieved within less than 1 week of cultivation, compared to a biomass concentration of 15.5 gl(-1) in normal batch culture after 2 weeks of cultivation. The yield is also increased from 0.32 g to 0.43 g biomass (g glucose)(-1). The strategy developed provides an effective method for the production of biomass of M. chlorophenolicum serving as the inoculum in remediation technologies.