The effects of the aromatic hydrocarbons benzene and toluene on Nitrosomonas europaea, a nitrifying bacterium that plays an important role in the removal of nitrogen from wastewater treatment plants, were studied in batch reactors. Exposure to 20 M toluene and 40 M benzene resulted in a 50% reduction in nitrifying activity after 1 h. However, Affymetrix microarray experiments detected no significant changes in gene expression in toluene exposed cells. Cells exposed to benzene were found to up-regulate a gene cluster (NE 1545 - NE 1551). This gene cluster appears to be involved with fatty-acid metabolism, lipid and membrane protein biosynthesis. TEM experiments reveal that cells exposed to benzene decrease the thickness of their membrane and the membrane becomes more structured. Keywords: stress response, benzene, toluene Overall design: 40 uM benzene caused 50 % inhibition in physiological response following 1 hour incubation. Transcriptional levels of the cells inhibited by benzene were compared with the cells under control conditions (ie, no benzene). 20 uM toluene also caused 50 % inhibition in physiological response following 1 hour incubation. Transcriptional levels of the cells inhibited by toluene were compared with the cells under control conditions (ie, no toluene).