The durations of a mitotic cell cycle and its component phases were investigated in the root meristematic cells of male Rumex acetosa by means of pulse labelling with ³H-thymidine autoradiography. The average duration of the mitotic cell cycle was determined to be 15.5 hours, and that of its component phases to be 2.4 hours for G₁, 7.8 hours for S, 3.5 hours for G₂, 0.9 hours for prophase, 0.5 hours for metaphase and 0.4 hours for ana- and telophase, respectively.