Although cooxidative biodegradation of mono-halogenated hydrocarbons has been well studied in the model NH3-oxidizing bacterium, Nitrosomonas europaea, virtually no information exists about cooxidation of these compounds by native populations of NH3-oxidizing bacteria. To address this subject, nitrifying activity was stimulated to 125-400 nmol NO3-produced g(-1) soil h(-1) by first incubating a Ca(OH)(2)-amended, silt loam soil (pH 7.0 +/- 0.2) at field capacity (270 g H2O kg(-1) soil) with 10 Enrol NH4+ g(-1) soil for 14 days, followed by another 10 days of incubation in a shaken slurry (2:1 water:soil, v/w) with periodic pH adjustment and maintenance of 10 mM NH4+. These slurries actively degraded both methyl bromide (MeBr) and ethyl chloride (EtCl) at maximum rates of 20-30 nmol ml(-1) h(-1) that could be sustained for approximately 12 h. Although the MeBr degradation rates were linear for the first 10-12 h of incubation, they could not be sustained regardless of NH4+ level and declined to zero over 20 h of incubation. The transformation capacity of the slurry enrichments (similar to1 mumol MeBr ml(-1) soil slurry) was similar to the value measured previously in cell suspensions of N. europaea with similar NH3-oxidizing activity. Several MeBr-degrading characteristics of the nitrifying enrichments were found to be similar to those documented in the literature for MeBr-degrading methanotrophs and facultatively methylotrophic bacteria.