The infectious agent of bacterial gill disease is Flavobacterium branchiophilium, a yellow-pigmented, Gram-negative, nonmotile, rod-shaped bacterium. Flavobacterium branchiophilium is highly contagious to salmonids with the bacteria readily adhering to gill tissue. Bacterial gill disease has a broad salmonid host range and worldwide distribution; basically the disease can be found almost everywhere salmonids are intensively culture. Histopathology will show epithelial hyperplasia, clubbing and fusion of gill lamellae, and accumulations of long, thin Gram-negative bacteria. The presence of long slender, flexing bacteria in typical “haystack” aggregations in wet mounts of affected gill tissue or other lesions supports the presumptive diagnosis. A typically chronic benign infectious disease characterized by cysts in the gill epithelium of wild or farmed fish. Numerous viral, bacterial and parasitic diseases cause similar lesions that are limited to the gill tissue. The diagnosis is based on clinical signs, macroscopic and microscopic examination of the gill tissue.