A disease outbreak occurred during the Spring of 2006 among rainbow trout, Oncorhynchus mykiss, kept in growout tanks. During the outbreak 35% of the rainbow trout died. Typical clinical signs were ulceration on the dorsal surface of the fish. Based on biochemical characteristics and sequence of 16S rRNA, the causative bacteria isolated from diseased fish were identified as Pseudomonas putida. Histologically, skin ulcer initially was characterized by sloughed off epithelia and epithelial necrosis. In the advanced form of the disease, almost all layers of the skin down to the epidermis were lost. Outside of Japan, this study reports the first P. putida infection in fish. (c) 2006 Elsevier B.V. All rights reserved.