The toxicity and horizontal transmission of thiamethoxam was evaluated in the workers of the Asian subterranean termite Coptotermes gestroi Wasmann (Isoptera: Rhinotermitidae). Brief exposure to sand treated with thiamethoxam at concentration ranging from 0.25 to 50 µg/ml resulted in a dose-dependent mortality in C. gestroi. Sand treated with 50 µg/ml thiamethoxam resulted in very high mortality within 30-60 min of exposure. Termites exposed to sand treated with 0.25-25 µg/ml exhibited delayed toxicity and nonrepellency in C. gestroi. A horizontal transmission study using 25 µg/ml of thiamethoxam at donor-recipient ratio of at least 2:5 (treated:untreated) indicated that thiamethoxam can be transferred between exposed and unexposed workers, resulting in significant termite mortality in unexposed termites within 1-3 d post exposure.