The cell-cycle duration and the growth fraction were estimated in the vegetative shoot apical meristem of Sinapis alba L. The length of the cell cycle was about 86 h, i.e. 2.5 times shorter than the cell-doubling time (M. Bodson, 1975, Ann. Bot. 39, 547-554) and the growth fraction was between 32 to 41%. These data demonstrated that the cell population of this meristem was heterogeneous, including one subpopulation of rapidly cycling cells and one subpopulation of non-cycling cells, i.e. cells with a very long cell cycle compared with that of the rapidly cycling cells. Non-cycling cells had no particular localization within the meristem. Both the central and peripheral zones of the meristem were mosaics of rapidly cycling and non-cycling cells.