Feeding stimulants for the tiger puffer Takifugu rubripes were identified by omission test using the synthetic clam Tapes japonicus extract, based on a daily feeding rate for casein diet with each test solution. The amino acid fraction showed remarkably higher feeding stimulant activity than those of nucleotide and other chemical fractions in the extract. Among 18 chemicals in the amino acid fraction, L-serine, L-aspartic acid, glycine, and L-alanine showed slightly higher activity than deionized water, but lower than the amino acid fraction. The mixtureof the above four amino acids plus betaine showed a markedly higher feeding stimulant activity than that of the synthetic extract, indicating the synergistic effect. The positive supplement level of the four amino acids plus betaine was foundto be that corre-sponding to 100g of clam muscle per 100g of casein diet.