The effects of different sowing dates on the growth and development in yam bean (Pachyrhizus erosus Urban) were investigated with reference to the photoperiodic changes during the plant cycle. Whatever the sowing dates, tuberization was initiated between 4 and 6 weeks after sowing. Floral induction occurred under short days. When the whole cycle of the plant took place under short days, tuberization and flowering processes started at the same time. Plants under those conditions (October and December sowings) presented a reduced shoot due to the fact that tuber bulking competed with the development of the aerial parts. The cycle duration did not exceed 5 months. Plants which ran their vegetative phase under long days (May plants) delayed the tuber growth which occurred only after the development of a large and vigorous foliage resulting from vine production. The cycle duration could reach 8 to 9 months. February plants which began their cycle under short days but did not achieve it before going through the long day influence, presented successively short and long day pattern of development.