Trifluralin (2, 6 dinitrc-N N-dipropyl-4-trifluoro-methylaniline) severely retarded the secondary root development of seedling cotton at very low concentrations in water, sand, and soil culture in the glass house. Although retarded, the seedlings survived at concentrations of 5.0 p.p.m., the highest rate used. In water culture length of secondary roots was reduced at 0.01 p.p.m. In sand culture a concentration of 0.125 p.p.m. significantly reduced the root weight, but in clay-loam soil a concentration of 0.25 p.p.m. was needed to produce the same degree of reduced secondary root growth. No statistically significant retardation in total root growth was obtained when the cotton roots were able to grow in an untreated layer of sand or clay-loam soil, when the trifluralin treated layer was placed either above or below the untreated layer.