The present knowledge about rainbow trout fry syndrome (RTFS), a serious bacterial disease affecting cultivated rainbow trout (Oncorhynchus mykiss) fry and fingerling in Europe is reviewed. The aetiologic agent of this disease is Flavobacterium psychrophilum, a Gram negative, filamentous, chromogenic rod shaped bacterium. The disease usually occurs in the spring when water temperatures are below 10°C. Gross pathological signs of the disease include lethargy, darkening of the skin, bilateral exophthalmia, abdominal distension and periocular hemorrhaging. Internally ascites, swollen spleen, pallor of the liver and anorexia can also be observed. Histologically the most consistent and prominent changes occur in the spleen characterized by peripheral layering due to fibrous infiltration, loss of border definition and generalized intracellular oedema. The diagnosis of RTFS is generally confirmed by isolation of the bacterium from internal organs of fish showing the characteristics signs of RTFS. No effective control methods for RTFS are currently available to fish farmers. The control of RTFS largely depends on the use of antibiotics, such as oxytetracycline and amoxycillin.