Since ß-lactam antibiotics were first employed for use in human health in the 1940’s, bacterial pathogens resistant to these drugs have emerged. More recently, however, it has become clear that antibiotic resistance is not solely the result of human-mediated use. Isolates of bacteria from a variety of environmental sources reveal surprisingly widespread levels of resistance as well. Thus, the question arises whether resistance observed in clinical versus environmental settings are drawn from the same reservoir or, rather, comprise distinct gene pools. One ß-lactamase gene, ampC, from one species of enteric bacteria, Enterobacter cloacae, was chosen for study. DNA sequences were obtained for isolates from clinical and environmental settings and their phylogenetic relationships explored to better understand the levels and nature of ampC resistance variation and the relationship between the clinical and environmental ampC gene pools. These data were also used to address the protocol for naming the resistance genes. The results suggest that both clinical and environmental sequences share a similar evolutionary history and that they may have evolved under similar evolutionary pressures. Results also indicate that the sequences used in this study should all be given the same gene name.