Foliar sprays of calonyctin, a natural plant growth regulator extracted from the leaves ofCalonyction aculeatum (L.) House, at a concentration 0.05 activity units, increased the tuber yield of six potato (Solanum tuberosum L.) cultivars by 7–19% (mean 15%) during 1993 and 1994. Physiological differences between calonyctin-treated and untreated potato plants were usually detected 30–40 days after spraying them with calonyctin. The chlorophyll content of treated potato leaves increased, and the chlorophyll a:b ratio decreased. By 40 days after treatment the root oxidizability and soluble protein contents of different parts of the plants also increased. Their subsequent decline as the plants aged was inhibited by calonyctin. It is concluded that calonyctin delayed the senescence of potato plants and increased tuber growth at later stages of development.