Trifluralin (2,6-dinitro-N, N-dipropy1-4- (trifluormethyl) benzenamine) was applied preemergent to soybean in plots drained or nondrained, in Louisiana. Plots 14.6 ㏊ were arranged to give 1683 g/㏊ of trifluralin. The half life of trifluralin in the top 15㎝ of soil was 42.6 days and 46.0 days in nondrained plot and drained plot, respectively. The concentrations of trifluralin in surface runoff water and subsurface runoff water were 0.62 ng/mL-0.02 ng/mL and 0.06 ng/mL-0.02 ng/mL, respectively. The concentration of trifluralin in runoff water was smaller than 2 ng/mL for trifluralin of U.S. Environmental Protection Agency advisory. Total loss of trifluralin in runoff water was 0.021% of applied amount at drained plots during three month after application. Trifluralin was moved hardly in the water. Subsurface drainage reduced trifluralin losses because concentration of trifluralin in the subsurface runoff water in drained fields was low.