Growth of the sea hare Aplysia kurodai was measured on chemically defined artificial diets, each diet deficient in 1 of 20 amino acids. In addition, a diet complete in all nutrients, a diet deficient in all amino acids, a diet of the green alga Ulva pertusa, and a control diet of agar and cellulose fibre alone were tested, and a group of animals was starved. As expected, the sea hares lost weight on the control diet of agar and cellulose, on the amino acid deficient diet, and under conditions of starvation, and gained weight on the Ulva pertusa diet. The diet complete in all nutrients and 18 of 20 diets deficient in a single amino acid maintained steady weight in A. kurodai over the 24-day experimental period (only leucine- and valine-deficient diets showed significant weight losses). If the first 4 days are omitted and only differences between weights on day 4 and day 24 are considered (many animals lost weight only at the start of an experiment), then none of the artificial diets showed significant changes. No correlation was found between rates of consumption of the different amino acid deficient artificial diets and weight change, nor was there correlation between spawn production or mortality, and presence or absence of "essential" amino acids.