In vitro tuberization responses of eleven clones of potato were evaluated under different photoperiods and light intensities. No growth regulators were added to the medium to avoid the potential modification of the response to the environmental stimuli.Plantlets were grown on MS medium containing 3% sucrose for four weeks under long days (16 h). Tuberization was induced by short days (8 h). Five photoperiodic combinations and three light intensity treatments were applied.Light applied after the tuber induction phase delayed or inhibited tuber initiation (at proper photoperiods both at 111 and 55 ~mol m 2 s 1 light intensities at 24/15 ~ day/night temperature). Darkness following the induction stage accelerated and synchronized tuber initiation after high light intensity. However, these effects of dark treatment depended on light intensity applied in short days before dark treatment. There was an interaction between the duration of darkness applied in photoperiodic treatments and the light intensity applied in short days on tuber initiation. No relationship was observed between the maturity groups of clones tested and their tuber initiation responses. Tight interaction (P<0.01) of treatments and clones was proved by statistical analysis. Reliable tuberization and high rate of tuberization (1.20-1.52) can be reached using these hormon-free systems.