Fibrous root cytokinin activIty in Dahlia variabilis was examined in relation to tuber development both under long-days and short-days. Root cytokinin activity increased during tuber initiation but decreased thereafter. The initial high cytokinin levels may benefit early events of tuberization, namely cell division and differentiation, of tuberforming roots which were placed immediately above roots at the cotyledonary node. Subsequent events of tuber growth, accumulation of dry matter, may have benefited by low root cytokinin levels which influence the shoot hormonal composition leading to preferential translocation of dry matter into tubers.