The chick urinary metabolites of chloroethylthiamine, a potent coccidiostat, were studied. After 20mg of ³⁵S-chloroethylthiamine was administered to chicks, 17 radioactive metabolites were separated by Amberlite CG-50 column chromatography followed by cellulose thin layer chromatography. Five major metabolites were identified to be unchanged chloroethylthiamine, thiamine, thiamine anhydride, thiamine anhydride sulfoxide and thiamine anhydride sulfone. The amounts were 34.2, 3.3, 18.3, 4.9 and 4.1% of the total urinary radioactivity, respectively. After intraperitoneal administration of ³⁵S-chloroethylthiamine, however, the formation of thiamine was negligible. These results suggest that the gastrointestinal tissue or the intestinal flora might play an important role in the biotransformation of chloroethylthiamine to thiamine. It was indicated that the stimulative effect of chloroethylthiamine on chick growth is due to the formation of thiamine.