Changes in soluble carbohydrates and PAGE pattern of tap root proteins were monitored in yam bean [Pachyrhizus erosus (L.) Urban] tap root under inductive (short days) and non-inductive (long days) environments. Up to five weeks after sowing (WAS), tap roots contained essentially sucrose. Under the short-day environment, a neutral invertase (NI) activity appeared from 6 WAS, concomitantly with glucose and fructose accumulation. Under the long-day environment, soluble carbohydrates as well as the pattern of proteins remained unchanged throughout root development. From these data it is concluded that NI controls the accumulation of hexoses in tap roots and is regulated by short days.