The time of the mitotic cycle and of its component phases in meristematic root-tip cells ofHaplopappus gracilis(n= 2) has been determined by labeling with tritiated thymidine and following radioautographically the course of distribution of the tagged DNA. The total duration of the mitotic cycle was found to be 10.5 hours, which included a synthetic period of 4.0 hours, a post-synthetic growth period (G2) of 1.44 hours, prophase of 1.06 hours, metaphase of 0.29 hours, anaphase plus early telophase of 0.26 hours, and late telophase plus the pre-synthetic growth period (G1) of 3.46 hours.