Summary:The herbicide mefenacet has recently been developed for pre–emergence control of graminaceous and some dicotyledonous weeds in rice. Whereas mefenacet is known to inhibit root growth, its effect on the components of growth, cell division and cell enlargement, have not been determined. The purpose of this study was to evaluate the influence of mefenacet on growth, cell enlargement and cell division inChlamydomonas(a green alga) and oats. Cell division inChlamydomonaswas completely inhibited and cell enlargement was inhibited by an average of 18% during the first cell cycle period when cells were treated with 6–7 μM mefenacet. The rate of oat root growth in sand culture between 12 and 24 h after initiation of treatment was reduced 90% at 10 μM mefenacet. Cell enlargement of oat coleoptile sections in culture solution was inhibited 25% at 10 μM. Mefenacet inhibited mitotic entry rather than disrupted the mitotic sequence. Cell division in oat roots was inhibited 54% by 10 μM mefenacet. This shows cell division inhibition occurs in G1, S, or G2of the cell cycle. The action of mefenacet is most like that of the chloroacetamides among the herbicide classes known to inhibit