Malnutrition in fish population, although recognized by the earliest workers in fish culture, has only recenty received intensive study commensurate with that in other fields of animal nutrition. Recent experimental test diets have produced specific deficiency syndromes for most of the vitamins and have indicated the possibility of developing specific deficiencies for amino acids, fatty acids, and other basic nutrients. During the past 3 years the first evidence of susceptibility to bacterial disease in lots of fish with sub-clinical manifestations of vitamin deficiencies has opened an entire new field in fish-disease study.