Proliferative gill disease (PGD) has become a significant problem in commercial catfish operations in the southeast and in California Nine cases were presented to the LSU-SVM Aquatic Animal Diagnostic Lab in the past year with significant mortality. At present, there is controversy over the etiology of PGD. Ammonia or other environmental toxins, Henneguya and an un-described microsporidean have all been implicated, but no definitive information is available. Information will be presented on the etiology, epizootiology and patholgenesis of PGD in Louisiana using both naturally and experimentally infected fish. Light and electronmicroscopic observations will be presented, including sudan black & non-specific eaterase stains.