Nitrification by autotrophic ammonia-oxidizing bacteria, such as Nitrosomonas europaea, is a key step in the global nitrogen cycle and is widely used for the removal of ammonia and cellular nitrogen in wastewater treatment facilities. The enzyme responsible for the initial step of nitrification, ammonia monooxygenase (AMO), which oxidizes ammonia to hydroxylamine, is a reasonably non-specific enzyme which acts on a variety of different substrates. This non-specificity allows for AMO to have inhibitory effects from a wide variety of compounds, including chlorobenzene. The work presented here is one portion of a larger study aimed at identifying sentinel genes in N. europaea, genes that are significantly up- or down-regulated as a result of the presence of specific nitrification inhib itors. In this study, chlorobenzene concentrations of 2 ìM inhibited nitrite production in batch cultures of N. europaea by approximately 50%. Based on oxygen uptake, chlorobenzene was found to inhibit AMO activity, while not significantly inhibiting the downstream process of hydroxylamine oxidation to nitrite. Toxicity was implied by further loss of activity under longer duration exposures. Two-dimensional protein gels were compared and putative response proteins were identified and sequenced. Whole genome microarrays were hybridized to labeled cDNA produced from mRNA harvested from batch culture experiments and analyzed for differential gene expression. Protein and transcription results will be compared for identification of potential sentinel genes indicating chlorobenzene inhibition of nitrification in N. europaea.