In bioremediation of petroleum-contaminated soils the ability to detect and quantify genes directly involved in biodegradation is an important tool for assessing the impact of remediation technologies on microbial populations and the remedial potential of the site.  Oxygenase genes are often chosen as a target because of their substrate specificity and they are often the rate-limiting step in aromatic hydrocarbon biodegradation.  Researchers have suggested that monitoring gene expression (i.e., mRNA) rather than gene numbers (i.e., DNA) is a more informative measure of site assessment, because of the linkage between mRNA and specific metabolic activities. Before testing under field conditions, in the present study we quantified gene numbers (DNA) and compared them to gene expression (RNA) in seven strains of known aromatic hydrocarbon (e.g., naphthalene, biphenyl, toluene and xylene) degrading bacteria.  Results from one strain, Pseudomonas putida G7 is likely common to all strains.  Oxygenase gene (NAH) quantities (DNA) per cell did not differ regardless of the growth phase at which bacteria were sampled.  Whereas expression of the oxygenase gene per cell increased significantly during exponential growth reaching a maximum at mid to late log phase and then decreased rapidly during stationary phase.  Both measures are of value for evaluation of remediation sites.  DNA quantities correlated with cell number providing a good indicator of the presence of biodegraders in a site.  Gene expression correlated with cell growth serving as an indicator that active degradation was occurring.