The phytotoxicity and the persistence of ethyl N,N-dipropylthiocarbamate (EPTC), S-propyl butylethylthiocarbamate (pebulate), ethyl N-ethyl-N-cyclohexylthiocarbamate (hereinafter referred to as R-2063), and S-2,3-dichloroallyl N,N-diisopropylthiolcarbamate (diallate) were studied in nutrient solution and in five soil types. Both in nutrient solution and in soil, EPTC was the most toxic to barnyardgrass [Echinochloa crusgalli (L) Beauv.], followed in order by pebulate, R-2063, and diallate. Herbicidal injury, as reflected by the morphological aberrations that occurred in treated plants, was much more severe than the reduction in fresh weight. Thus, toxicity studies based on plant height had greater validity than those based on weight. The initial toxicity of each herbicide was high in those soils low in organic matter and low in soils high in organic matter. The persistence of the four thiocarbamates in the soil was associated primarily with adsorptive forces and microbial degradation. In a soil with extremely large or small adsorptive capacity, there was rapid inactivation of the herbicides because of strong adsorption to the colloids or lack of adsorption with consequent loss by volatilization. Under the latter conditions, the microbial breakdown played only a secondary role. The herbicides persisted longer in soils with more moderate adsorptive capacities but were subjected to more striking degradation by microorganisms. The patterns and rates of inactivation of the four herbicides in a given soil were quite similar. A particular concentration of each herbicide stimulated growth in barnyardgrass, as reflected both in the fresh weights and in the heights of the plants. The nature of this stimulation was not determined.