In this study a possible role for batatasin-type phenolic growth inhibitors in the control of dormancy and, therefore, the storage life of tubers of Dioscorea alata and D. esculenta, two economically important food yams, was investigated. After extraction, the level of endogenous growth inhibitory phenolics was estimated by a wheat coleoptile growth test and batatasins specifically determined by fluorescence spectroscopy and gas-liquid chromatography. There was a gradual decrease in growth inhibitory phenolics in tubers of both species during dormancy and in D. alata this closely paralleled a decrease in batatasin content. It was found that batatasin-type growth inhibitory phenolics accumulated rapidly in developing tubers just prior to the onset of dormancy and were assyrnetrically distributed, being concentrated in the proximal (head) region and in the peripheral zone just beneath the periderm. Gibberellin A3 treatment produced a promotion of the dormant period and a correlative rise in the growth inhibitory phenolic level. Effects of maleic hydrazide and ethylene chlorohydrin are also reported. The results are discussed in terms of the mechanism of dormancy control in these tubers and it is concluded that endogenous batatasin-type phenolics are involved in this.