We investigated the effect of different oxygen regimes on growth patterns of Pseudomonas spp. during benzene degradation in microcosm batch studies. Benzene degradation was induced by limiting oxygen available for microbial activity, which consists of three initial-dissolved oxygen (DO) levels of oxic, hypoxic, and anoxic conditions. Batch experiments were performed for cell growth and benzene degradation by inoculating three strains of Pseudomonas spp. (Pseudomonas aeruginosa, Pseudomonas fluorescens, Pseudomonas putida) in mineral salt medium containing aqueous benzene. Results showed that all strains were capable to grow and degrade benzene under all oxygen regimes but in a different manner. The highest cell growth of P. aeruginosa and P. fluorescens was achieved under oxic and anoxic condition, respectively, but there was no substantial difference on benzene degradation between the oxygen treatments with about 25% reduction for both strains. P. putida showed a facultative process for both cell growth and benzene degradation. This reveals that care should be taken in selection of microorganisms with regard to environmental studies since they exhibit different responses for given environmental conditions such as DO levels.