Kidney disease is one of the most puzzling fish diseases known to exist in the United States. In less than Io years it has invaded the Pacific Northwest, exacting a heavy toll of hatchery salmon. Its first appearance apparently was in Massachusetts where Belding and Merrill' described a disease similar to that now seen on the Pacific Coast. In I946 it was diagnosed in Washington2 and since that time has been observed in an ever increasing number of hatcheries. There are unpublished reports of the same or similar diseases in both California and Washington in the early I93o's.3 The latest outbreaks occurred in the Federal hatcheries at Berlin, New Hampshire, and Cortland, New York, in brook, brown, and rainbow trout.4 There is evidence to indicate that the disease may be much more widely spread in New York State.5 The disease is especially dangerous since little is known of the origin or source of the causative agent. Indeed, the classification of the diplobacillus associated with kidney disease is still uncertain. Thus, with our present knowledge, it is difficult or impossible to eradicate the malady from an infected hatchery. Histopathologic studies were undertaken to clarify the pathology of the disease and to compare the eastern form with the western form.