Enteric redmouth disease (ERM) was first recognised as a serious infection of salmonid fish in the 1950s at a hatchery in the Hagerman Valley, Idaho, USA. It is caused by the bacterium Yersinia ruckeri and is an economically important disease, principally affecting farmed rainbow trout. However, the causal agent has been shown to be increasingly widespread in a variety of salmonid and other fish species, in many countries around the world. In the early stages of ERM, fish appear lethargic, dark in colour, and often exhibit a wasting condition due to anorexia. There may also be a characteristic reddening of the mouth, tongue, jaw area and operculum. Internal signs can be characterised by petichial haemorrhages in various tissues and the intestine can be filled with a yellow, mucoid material. Stress factors such as rising water temperatures, handling, etc. play an important role in the disease course. Control of ERM is normally achieved by good husbandry, immunization and the use of antimicrobial agents.