Pendimethalin exerts a marked depressive effect on mitosis in the root meristem of <em>Allium cepa</em> L. and disturbs chromosome division. The depressive effect starts at a concentration between 10 and 1 µM and the functional disturbance can be observed at 0·1 µM. The effects of treatment also show in inhibition of root growth and thickening of the meristematic area.