Feeding stimulants for the common japanese goby Acanthogobius flavimanus were examined by omission test using the synthetic krill Euphausia pacifica extract, based on a daily feeding of casein diet with various test solution. The complete synthetic extract of krill possessed a remarkable feeding stimulant activity. Subdivision of the synthetic extract showed that the group of neutral amino acids plus nucleotides was highly active. However, the individual amino acids, nucleotides and other bases groups as well as the groups of amino acids plus other bases and nucleotides plus other bases were inherently lower active. Therefore, the feeding stimulant activity of synthetic extract of krill for the fish was considered to be due to the synergistic effect of neutral amino acids plus nucleotides.