An immunohistochemical method is described for the detection of Flexibacter psychrophilus in formalin-fixed, paraffin-wax-embedded fry of rainbow trout. Rabbit antiserum as well as rainbow trout hyperimmune serum were used in the study. The distribution and tissue localization of the bacterium was compared in naturally and experimentally (intraperitoneal injections) infected fry by use of immunohistochemistry. This study showed that F. psychrophilus could be detected in paraffin-wax-embedded tissue of rainbow trout fry by immunohistochemistry. The principal immunohistochemical findings in naturally and experimentally infected fry showed that there was a localization of bacteria in the monocyte-macrophage system, in skin lesions, and in the retina and the choroid gland of the eye. The dermal changes included superficial or deep ulcers extending to the subcutaneous tissue or the musculature accompanied by inflammatory cell infiltrates in which polymorphonuclear inflammatory cells were shown to contain the bacterium in the cytoplasm by immunostaining. The eye changes were likewise a common finding in chronic cases with severe inflammatory changes in the retina and with numerous bacteria in inflammatory (mainly polymorphonuclear) cells. F. psychrophilus infection in rainbow trout fry involves the monocyte-macrophage system extensively, and the concurrent localization of bacteria in the skin ulcers and retinal inflammation points to the probable involvement of the bacterium in the development of the lesions which are typically found during the chronic stage of the disease.