The bioactivity of acetochlor (2-chloro-N-(ethoxymethyl)-6′-ethyl-o-acetotoluidide) and metazachlor (2- chloro-N-(1-pyrazolylmethyl)-aceto-2′,6′-xylidide) was evaluated in thirty-two soils under controlled conditions. Grain sorghum (Sorghum bicolorL. Moench) cultivar NK222 was used as the indicator plant in the bioassays. Bioactivity was correlated with organic matter content, clay content, cation exchange capacity, P reversion and pH of the soils. Organic matter was most closely related to the bioactivity of both herbicides. Clay content was a significantly poorer predictor of the bioactivity of both herbicides than was organic matter content. Inclusion of soil pH with organic matter in a multiple regression model significantly improved the prediction of metazachlor bioactivity. Soil pH was not associated with the bioactivity of acetochlor.