Glyphosate has been widely used to control annual, perennial, and biennial weeds including Conyza species. Conyza sumatrensis (Sumatran fleabane) is considered a highly invasive and troublesome weed worldwide, including in European and Mediterranean regions. In Turkey, the use of glyphosate in orchards has recently increased; however, extensive use of glyphosate has resulted in poor control of C. sumatrensis in several peach orchards. The objectives of this research were to determine if C. sumatrensis is resistant to glyphosate and identify alternative herbicides with different modes of action that can be used instead of glyphosate. Two dose response studies were conducted in the greenhouse to evaluate the response of four C. sumatrensis populations to glyphosate, chlorsulfuron, and metribuzin. Glyphosate isopropyl amine and glyphosate potassium was applied at 0, 0.25, 0.5, 1, 2, 4, and 8 times the use rate of 1080 g a.e./ha (a.e. indicates acid equivalent) when the plants were at rosette (5-6 true leaves) and vegetative (20-22 cm tall) stages. Effects of both glyphosate formulations were combined. The resistant populations showed higher resistance 3.8 to 6.6 and 5.3 to 7.8 times at rosette stage and vegetative stage, respectively, compared with the susceptible population. Furthermore, glyphosate-resistant populations were treated with chlorsulfuron and metribuzin at 0, 0.25, 0.5, 1, 2, 4, and 8 times use rate of 7.5 and 350 g a.i./ha, respectively at the rosette stage. The glyphosate-resistant populations exhibited 2.4 to 3.8 times more resistance to chlorsulfuron, but were adequately controlled with metribuzin.