The duration of the mitotic cycle and its phases was measured in the root tips ofOrnithogalum umbellatumby autoradiographic technique, after labelling with3H-thymidine. The average total duration of the mitotic cycle was found to be 14 h. This time include: a synthetic period (S) of 8 h, a post-synthetic period (G2) of 5 h 29′ and a mitotic duration of 40′. The main feature ofOrnithogalum umbellatummitotic cycle seems tobe the lack of G1. The single phases duration was also considered as percentage of the total time of the cycle.