Multiple-seeded cocklebur (Xanthium strumarium L.) is a biotype which has different morphology and higher seedling production ability than common cocklebur. Greenhouse studies were conducted to investigate the bioherbicidal activity of Alternaria helianthi (Hansf.) Tubaki and Nishih. on multiple-seeded cocklebur as affected by various rates of Tenkoz COC (crop oil concentrate), Activator 90 (non-ionic surfactant), BAS 9050 0 S (methylated oil), Silwet L-77 (organosilicone surfactant) and Top film (natural based surfactant). Taking X as the recommended rate for each surfactant, 0-X, -X, -X, X and 2-X rates were used for each of the surfactants. Surfactants were added to the conidial suspension of A. helianthi. Each surfactant rate was also applied with sterile water without any A. helianthi spore. Treated plants were kept in the dew chamber for 6 h before transferring to the greenhouse. At the end of the experiment (two weeks after treatment), plant shoots were clipped at the soil surface and the fresh weights were determined. Alternaria helianthi resulted in significant reduction in fresh weight of multiple-seeded cocklebur as compared to the plants treated without the fungus. Among five surfactants, Activator 90 and Silwet L-77 had significant effects on the bioherbicidal activity of A. helianthi in reducing fresh weight of multiple-seeded cocklebur. Fresh weight decreased with increase in surfactant rates. Our data demonstrate that A. helianthi control multiple-seeded cocklebur more with higher rates of Activator 90 and Silwet L-77 .