Certain soybean varieties flowered, but did not set fruit under 20-hour photoperiods. Three short (12-hour) photoperiods applied 1 or more weeks after the beginning of flowering induced fruitset which continued throughout the flowering period. One short day was not sufficient. Three short days given to vegetative plants before flowering induced earlier flowering but not fruitset. Short-day treatments applied at the first sign of flowering were also ineffective. Flowering but non-fruiting long-day plants did not produce normal anthers of viable pollen. Individual flowers set fruit if fertilized with viable pollen from plants grown continuously under short days.