Simazine is a pre-emergent herbicide of low water solubility (5 ppm) that is long-lasting and has high crop selectivity. Simazine residues in a citrus orchard, portions of which have been under chemical weed control for an extended period, were quantified. The orchard is located in the semi-tropical region of South Texas on alluvial Hidalgo sandy clay soil of relatively uniform profile. The orchard investigated had been included in a long-term weed control study lasting 30 years. Three treatments were established based upon the history of the herbicide use. Treatment A had received herbicide applications for 30 years and simazine applications during the last 8 years (22.8 kg·ha-1 total simazine application). Treatment B was mechanically cultivated without herbicides for 28 years and received annual simazine applacations the last 2 years (7.6 kg·ha-1 total). Treatment C was maintained in grass sod and received no herbicide. Soil fractions were taken at 15-cm increments by soil probe to a total depth of 1.5 m. Three probes in each of three replicate plots for each treatment were analyzed. The highest concentration of simazine was found near the surface and decreased with depth. The rate of this decrease was less with treatment A than with B. In the depth range of 15 to 105 cm, treatment A had a greater simazine content than with treatment B. Simazine was detected in some samples below the 120 cm depth.