The effect of waterlogging the vines of two yam varieties for 24,48 and 72 hours at two stages of growth, with or without fertiliser application, was studied in the field.Waterlogging caused a progressive degeneration of the leaf starting with the development of fresh lesions on the lower leaf surface, through necrotic spots or portions, to complete leaf necrosis. The degree of leaf damage was greater with Um 680 (Dioscorea d a t a ) than with Obiaoturugo (D. rotundata). Waterlogging also caused the breakdown of the apical buds of the vines. The degree of damage increased with the duration of waterlogging. Younger plants suffered more damage than older plants, and plants treated with fertiliser suffered more than plants without fertiliser. Waterlogging ultimately hastened the final senescence of the yam vine.Waterlogging vines for 24 h had no effect on tuber yield, while waterlogging for 48 and 72 h reduced tuber yield by 32.4% and 43.2% respectively (P< 0.01).Waterlogging vines at the early growth stage produced 47.6% (P< 0.01) less yield than waterlogging at a later stage.It is suggested that short-term or long-term waterlogging of the yam vine, or parts thereof, is the main cause of reduced leaf area and low yield in unstaked compared with staked yam crops. Waterlogging may also be a predisposing factor to disease infection of yam vines by soil-borne micro-organisms.