Triasulfuron (3-(6-methoxy-4-methyl-1,3,5-triazin-2-yl)-1-[2-(2- chloroethoxy)-phenylsulfonyl)-urea} is a new pre- and post-emergence herbicide developed for the control of broad-leaved_'Teeds and some grasses in small grain cereals at rates of 10-20gha . Oilseed rape, pea, broad bean, Senecio vulgaris, Veronica persica, Lolium renne, Poa annua, Poa trivialis and prates were severely a ed 5y pre-emergence and post-emergence applications of triasulfuron but wheat and barley were much more resistant. The development of injury symptoms was generally slow and was characterized by an initial growth retardation followed by chlorosis and necrosis with death occuring 3-4 weeks after application. The herbicide had a flat dose-response curve throughout the investigations. Wheat and barley showed greater tolerance to post-emergence than to pre-emergence applications. Pre-emergence applications of triasulfuron reduced tiller numbers in barley and wheat but application of the herbicide at the 2-3 leaf stage stimulated the outgrowth of tiller buds but this effect was temporary and was not the result of visible physical damage to the shoot meristem. There was no intra-specific variability between cultivars of wheat and barley. Grain yield and other yield components of spring barley were not affected by post-emergence treatments of triasulfuron. Triasulfuron had high activity through the soil and both the roots and subterranean shoots of developing seedlings absorbed the herbicide from the soil. The bioactivity and mobility of the herbicide down the soil profile was inversely related to the organic matter content of the soil. The amount and frequency of rainfall directly influenced the rate of leaching of the herbicide down the soil profile. Comparisons of the rates of disappearance of triasulfuron in autoclaved and non-autoclaved soils suggested the involvement of a biglogical degragation pathway. Triasulfuron disappeared faster at 30 C than at 10 C and higher moisture levels enhanced the rate of breakdown. Studies on the mode of action of triasulfuron indicated that the herbicide acts by inhibiting cell division in susceptible plants. Evidence suggested that the inhibition occurred during interphase rather than during the mitotic sequence. The addition of a 1: 1 mixture of isoleucine and valine to the treatment solution prevented the inhibition of cell division at the root tips of broad bean.