Effects of an environmental factor, the photoperiod, on tuberization were analyzed in vitro on 7 different potato cultivars from various maturity groups and genetic origin. No growth regulators were added to the culture medium, to preserve natural endogenous equilibrium of hormones. After culturing of shoots for 4 weeks under long days, tuberization was induced with pouring of 8% sucrose solution onto the cultures and five different photoperiodic treatments consisting of different combinations of short days and total darkness. Light (8 h) during tuber induction delayed tuberization, while the dark treatment (0 h) after short days (8 h) promoted a rapid tuber initiation. The beginning of tuberization could not be correlated with maturity groups. In vitro tuber formation has occurred at a rate of at least one tuber per plantlet and this tuberization rate were at least as high as any method previously described based on the addition of growth regulators. This would indicate, that photoperiod controlling tuberization processes in vivo, trigger a general state of induction in plantlets cultured in vitro.