Target site studies were undertaken to examine the difference in susceptibility of Formosan subterranean termite, Coptotermes formosanus Shiraki, workers and soldiers to chlorpyrifos. Workers exhibited significantly greater acetylcholinesterase activity per insect than soldiers (118.63 ± 48.51 versus 47.98 ± 22.59 mOD/min/insect equivalent). Likewise, enzyme activity (mean ± SD) per milligram of protein was greater in workers than soldiers (440.30 ± 267.43 versus 311.53 ± 149.83 mOD/min/mg protein). The enzyme of soldiers was more sensitive to the acetylcholinesterase (AChE) inhibitors eserine and chlorpyrifos-oxon than that of workers. The I₅₀s of chlorpyrifos-oxon were 2.66 and 4.59 nM for soldiers and workers, respectively, whereas the I₅₀s of eserine were 16.56 and 25.41 nM for soldiers and workers, respectively. The amount of protein was significantly higher in workers than in soldiers with mean values of 0.270 ± 0.102 and 0.154 ± 0.054 mg/insect equivalent, respectively. We suggest that the differential response of workers and soldiers to chlorpyrifos may be due to the difference in AChE sensitivity to inhibition and the amount of protein between them.