Field studies conducted in 1985 confirmed that an organic arsenical-resistant common cocklebur (Xanthium strumarium L.) population exists in three counties of the Coastal Plains region of South Carolina. The population was resistant to postemergence treatments of DSMA and MSMA, but not to postemergence treatments of bentazon, 2,4-DB, atrazine, fluometuron, acifluorfen, paraquat, glyphosate, chlorimuron, and imazaquin. Control of the resistant (R) population required 8 to 16 times more DSMA than the susceptible (S) population. In another experiment, the influrence of stage of growth on the response of R and S populations to DSMA was examined. DSMA was applied at 1.12, 2.24, and 4.48 kg/ha when common cocklebur plants were 11, 42, and 125 cm tall. The S population was controlled at all rates and at all growth stages. However, the R population was controlled only at the 11 cm stage with 2.24 and 4 .48 kg/ ha.