In the American alligator (Alligator mississippiensis) the torso is subdivided by a post-pulmonary septum. Previous studies of this septum have focused on the hepatic pump; by actively regulating the contractile state of a skeletal muscle band the alligator can control the position of the liver and post-pulmonary septum, and the ventilatory pressures within the pleural cavities. The purpose of this study was to explore the morphology of the post-pulmonary septum in Alligator mississippiensis, and to document the pressure coupling between the pleural and peritoneal cavities of this species. The morphological investigation utilized a variety of imaging modalities, gross dissection, and histology, which revealed the presence of skeletal muscle within the post-pulmonary septum. To analyze pressure coupling, sub-adult specimens of A. mississippiensis were anesthetized with isoflurane then differential pressure sensors were surgically implanted into the pleural and peritoneal cavities; the outputs of these sensors were simultaneously recorded with the exhalatory gas analysis, which documented the ventilatory pattern. Using an inversion table the anesthetized alligators were tilted head-up or head-down, which generated a gravitational gradient sufficient to displace the liver. To further explore the pressure coupling, Veress needles were surgically implanted into the peritoneal and pleural cavities; these needles allowed positive or negative air pressure to be applied to either (or both) of the body cavities. The results of the analysis demonstrate that the post-pulmonary septum of Alligator is capable of coupling the peritoneal and pleural pressures, and does so at a level comparable to what has been reported for the mammalian diaphragm; the results also demonstrate that our understanding of the ventilatory mechanics of Alligator are incomplete.