AbstractA strain of carrot cells (Daucus carotacv. Kintoki) grew exponentially in the presence of 2,4‐dichlorophenoxyacetic acid (2,4‐D, 1 mg/1) with a doubling time of about 2 days. When those cells were transferred to a medium lacking 2,4‐D, they continued to grow at almost the same rate for about a week. When the cells were again transferred to the auxin‐free medium, the rate of cell division gradually decreased. After the cell division had ceased, cells were returned to the ordinary 2,4‐D medium. A burst of cell divisions occurred after about 2 days. Timing of DNA synthesis and of mitosis suggested that the cells had been arrested at G1phase. In a medium containing indoleacetic acid instead of 2,4‐D, the auxin was rapidly degraded and the culture was similarly synchronized as in the auxin‐o