The gene expression and activity of (+)-δ-cadinene synthase during cotton development and in response to stress, as well as the spatial and temporal pattern of sesquiterpene biosynthesis, constitute one of chemical defense mechanisms in cotton plants. In order to explore the effects of omethoate on the cotton defense in relation to (+)-δ-cadinene synthase and gossypol, effects of omethoate treatments on activity of (+)-δ-cadinene synthase and gossypol content in cotton seedlings were investigated. Cotton seedlings treated with 400 mg L-1 omethoate exhibited a significant decrease in the specific activity of (+)-δ-cadinene synthase from 12 to 120 h after treating when compared to the untreated control; significantly lower (+)-δ-cadinene synthase activity was also observed in cotton seedlings treated with 200 mg L-1 omethoate from 72 to 120 h after treating; but for cotton seedlings treated with 100 mg L-1 omethoate, from 12 to 120 h after treating, no significant changes were observed for activity of (+)-δ-cadinene synthase. The gossypol content in cotton seedlings treated with 100, 200 or 400 mg L-1 omethoate for different time periods showed no significant changes compared to that of the control. These results indicated that the activity of (+)-δ-cadinene synthase in cotton seedlings in responses to exposure of omethoate at three concentrations for different time periods followed dose- and time-dependent responses to omethoate exposure. With omethoate as a chemical stress factor for cotton seedlings, the cotton defense in relation to the activity of (+)-δ-cadinene synthase in cotton seedlings may be affected by omethoate application. The gene expression and activity of (+) - δ-cadinene synthase during cotton development and in response to stress, as well as the spatial and temporal pattern of sesquiterpene biosynthesis, constitute one of chemical defense mechanisms in cotton plants. In order to explore the effects of omethoate on the cotton defense in relation to (+) - δ-cadinene synthase and gossypol, effects of omethoate treatments on activity of (+) - δ-cadinene synthase and gossypol content in cotton seedlings were investigated. mg L-1 omethoate exhibited a significant decrease in the specific activity of (+) - δ-cadinene synthase from 12 to 120 h after treating when compared to the untreated control; significantly lower (+) - δ-cadinene synthase activity was also observed in cotton seedlings treated with 200 mg L-1 omethoate from 72 to 120 h after treating; but for cotton seedlings treated with 100 mg L-1 omethoate, from 12 to 120 h after treating, no significant changes were obser ved for activity of (+) - δ-cadinene synthase. The gossypol content in cotton seedlings treated with 100, 200 or 400 mg L-1 omethoate for different time periods showed no significant changes compared to to that of the control. These results indicated that the activity of (+) - δ-cadinene synthase in cotton seedlings in responses to exposure of omethoate at three concentrations for different time periods followed dose- and time-dependent responses to omethoate exposure. With omethoate as a chemical stress factor for cotton seedlings, the cotton defense in relation to the activity of (+) - δ-cadinene synthase in cotton seedlings may be affected by omethoate application.