Dry matter produced and allocated to the tuber and aerial portions of yam plants (Dioscorea spp) were studied over 3 cropping seasons in the moist forest region of Nigeria, in an attempt to relate dry matter accumulation to vegetative maturity in the crop. The effect of duration of growth of yam on post-harvest dormancy was also studied. Cultivars differed both in the total amount and period of growth when most dry matter was accumulated. A higher proportion of dry matter was allocated to tubers in lower-yielding than in higher-yielding cultivars. It appears that a yam tuber is ‘mature’ almost as soon as it is formed.