The tuber induction of the sweet potato is known to occur under a wide variety of environmental factors which regulate the levels of endogenous phytochromones. Since exogenously applied cytokinins are known to promote stem swelling, their relationship to tuberization is of interest. Little comparative work has been reported with sweet potato tubers except Hojo's work which described the presence of the cytokinin activity in the tubers. We were thus interested in the isolation and determination of the structure and the levels of sweet potato cytokinins in connection with an explanation of the tuberization process. cis-Zeatin riboside was isolated and identified as its trimethylsilyl derivative from tubers of sweet potato by solvent extraction, ion-exchange chromatography, high performance liquid chromatography, Amaranthus betacyanin bioassay, and mass spectrometry. Differentiation of cis- and trans-isomers was made by measurement of the chromatographic retention time, which showed the active substance was the cis-isomer. The level of the endogenous cis-zeatin riboside in sweet potato tubers was approximately estimated by means of Amaranthus betacyanin bioassay to be of the order of 236 ng/kg as benzyl adenine equivalent. Langille suggested that this compound may be the tuber-forming stimulus, and if true, it would also play a causative role in the tuberization process of the sweet potato.