Yellow nutsedge (Cyperus esculentus L.) and purple nutsedge (Cyperus rotundus L.) are troublesome weeds of cotton (Gossypium hirsutum L.) in the southeastern United States. Glyphosate and MSMA applied in combination may be more effective in controlling these weeds than either herbicide alone. In field experiments, glyphosate at 0.2, 0.4, 0.6 and 0.8 kg acid equivalent (a.e.) ha-1 and MSMA at 0.6, 1.1, and 2.2 kg a.i. ha-1 were applied alone and in combination as topical treatments to yellow nutsedge (plants 10 to 20 cm tall) in glyphosate-resistant cotton (3- to 4-leaf stage). When each herbicide was applied alone, yellow nutsedge control increased as the herbicide rate increased. Yellow nutsedge was controlled by glyphosate at 0.4 and 0.8 kg ha-1 at 46 and 71%, respectively, and by MSMA at 1.1 and 2.2 kg ha-1 at 40 and 90%, respectively. Regression analysis indicated that control of yellow nutsedge with glyphosate in the field and greenhouse improved as the concentration of MSMA increased. Purple nutsedge control with glyphosate was improved with the addition of MSMA, but only when glyphosate was applied at less than 0.6 kg ha-1. Glyphosate was more effective in controlling purple nutsedge than yellow nutsedge, but MSMA was more effective in controlling yellow nutsedge than purple nutsedge. Glyphosate did not cause visible injury to cotton, but MSMA treatments alone and in combination with glyphosate caused stunting and stem reddening. Greater injury occurred from the combination of glyphosate and MSMA than from either herbicide alone, and injury increased as the concentration of glyphosate or MSMA in the mixtures increased. Further research is needed to evaluate strategies to reduce injury and to measure yield response.