Satisfactory fulfillment of the nutritional requirements of experimental fish under subchronic and chronic test conditions (such as carcinogenesis studies) is critical to the outcome of a study. Nutritional requirements of many coldwater, warmwater, and marine fishes have been determined in part by the use of semipurified diets. Generally, these diets were fed to small fish in controlled environment tanks with no supplemental food available. The control diets contained semipurified natural ingredients, or, when appropriate, purified (crystalline) nutrients and were formulated to meet the requirements (approximate) of the test species. We fed the fish representative diets to insure that their nutritional needs were met and that no adverse pathology would develop. We then modified these control diets by removing one or more of the purified nutrients to determine the essentiality of a dietary component. Later, other diets were formulated to contain different amounts of the essential nutrients, or one ingredient was substituted for another, so that the quantitative requirements could be decided. Thus we could determine the dietary requirements for protein, fat, carbohydrate, and bulk, and the qualitative and quantitative requirements for the individual amino acids, vitamins, fatty acids, and some minerals. Semipurified diets were later used in toxicity studies for the measurement of the effect of different levels of manufactured contaminants, such as insecticides, or natural toxicants, such as aflatoxin, on the well-being of fishes. Information on the preparation of semipurified diets for fishes, feeding techniques, and nutritional requirements as well as suggestions on care of small fishes in tanks appropriate for longer term carcinogenesis studies is provided.