AbstractThe resolved isomer of metolachlor, S‐metolachlor, was registered in 1997. New formulations based primarily on the S‐metolachlor isomer are more active on a gram for gram metolachlor basis than formulations based on a racemic mixture of metolachlor containing a 50:50 ratio of the R and S isomers. The labelled use rates of S‐metolachlor‐based products were reduced by 35% to give equivalent weed control to metolachlor. However, several companies have recently registered new metolachlor formulations with the same recommended use rates for weed control as S‐metolachlor. This research was done to compare the soil behaviour and the biological activity of metolachlor and S‐metolachlor in different soils under greenhouse and field conditions. Although Kd ranged from 1.6 to 6.9 across the five soils, there were no differences in the binding of metolachlor and S‐metolachlor to soil or in the rate of soil solution dissipation in a given soil. However, both greenhouse and field studies showed that S‐metolachlor was 1.4–3‐fold more active than metolachlor against Echinochloa crus‐galli (L.) Beauv. in five different soils and that S‐metolachlor was more active than metolachlor in three Colorado field locations. When the rates of metolachlor and S‐metolachlor were adjusted for S isomer concentrations in the formulations, there were no differences between the formulations in field, greenhouse or bioassay studies. Thus herbicidal activity is due to the S isomers, with the R isomers being largely inactive. Copyright © 2006 Society of Chemical Industry