Of all the diseases responsible for the losses in the hatchery, those caused by the microscopic one-celled organisms, the bacteria, are the most common and present the most serious problem to the hatcheryman. They are found at practically every trout and salmon hatchery during some period of the year. The symptoms of the diseases they cause are difficult to recognize. This in itself is a hazard because treatment, to be successful, must be applied during the early stages or, in cases where no method of treatment is effective, the infected fish must be destroyed before the disease spreads to other parts of the hatchery. Here we have a group of diseases practically universal in distribution, difficult to recognize, and hard to treat even in those cases where a remedy is known. Every hatcheryman should be well acquainted with them for efficient and intelligent control. There follows a brief summary of the present status of our knowledge of the individual diseases.