In a previous paper2), it was reported that ascorbic acid occurs in all fish tissues, and that its concentration in different tissues of a given fish varies over a wide range. In the present study, ascorbic acid synthesis by carps has been investigated by means of radioactive tracer techniques. The results obtained are as follows: (1) After injection of D-glucose-1-C14 or D-glucuronolactone-6-C14 into carps, the specific activities of the visceral ascorbic acid were determined. From the data in Table 1, it is evident that carps are able to synthesize ascorbic acid from the such precursors as D-glucose and D-glucuronolactone. Although the administration of Chloretone stimulated the ascorbic acid synthesis in carps from D-glucose, excretion of ascorbic acid was not detected. (2) The ascorbic acid recovered from the carps administrated with the labeled sugars was degraded to determine the activities of the constituent carbon atoms. The results (Table 3) show that D-glucose-1-C14 was converted to ascorbic acid labeled primarily in carbon 6, while D-glucuronolactone-6-C14 was converted to ascorbic acid labeled predominately in carbon 1. Therefore, it appears that carps possess the same mechanism of ascorbic acid formation as have been found in rats. (3) Both the ascorbic acid contained in intact fish tissues and the ascorbic acid labeled with C14 were identified to be L-form by paper chromatography (Figs. 1 and 2).