The leaching of α,α,α-trifluoro-2,6-dinitro-N,N,-dipropyl-p-toluidine (trifluralin) and 3,5-dinitro-N4,N4-dipropylsulfanilamide (oryzalin) with and without surfactants was studied in soil columns. In dry soil without surfactants leaching of trifluralin was less than that of oryzalin and did not amount to more than a few centimeters despite the use of light soil and large amounts of herbicide. In wet soil trifluralin leached more than twice as deep as in dry soil with the same amount of water. The addition of surfactants at a concentration of 2% of the spray volume increased both depth of water penetration and herbicide movement into the soil. The most pronounced effect of surfactants was the increase in movement of trifluralin in dry soil, but they also increased trifluralin movement in wet soil. There was no direct relationship between the ability of the surfactants to improve water penetration and their effects on leaching of the two herbicides. It was therefore concluded that increased leaching was due to an effect of surfactants on the sorption-desorption balance rather than on improved penetration of water. The improved movement of trifluralin into the soil as a result of surfactant action was also evident from another experiment in which surfactants increased the herbicidal activity of trifluralin sprayed preemergence on the soil surface.