By using a no-choice feeding bioassay, delayed toxicity and concentration-dependent mortality of two chitinolytic enzyme inhibitors, pentoxifylline and psammaplin A, were evaluated by determining LT₅₀, LT₉₀, and LT₉₉ (lethal time) against the economically important eastern subterranean termite, Reticulitermes flavipes (Kollar). Pentoxifylline- and psammaplin A-incorporated diets (filter paper) were assayed at 0.01, 0.02, 0.04, 0.08, and 0.21% and 0.0375, 0.075, 0.15, and 0.3% active ingredient (wt:wt), respectively. Acetone-only treated filter paper served as diet for the control treatments. Termite workers were allowed to feed on diet until 100% test population mortality occurred (80–95 d). Both chitinase inhibitors were shown to be toxic to R. flavipes. Concentrationdependent toxicity occurred within the pentoxifylline treatments over the range of 0.01–0.08%, with 0.08% treatments producing an LT₅₀ of 32.2 d. However, mortality in response to psammaplin A treatments lacked concentration-dependent toxicity. Treatment with 0.3% psammaplin A produced an LT₅₀ of 21.3 d. Mortality in response to lower psammaplin A treatments displayed no concentration-dependent trends. This study provides the first report on delayed toxicity of chitinolytic enzyme inhibitors against eastern subterranean termites (order Isoptera) and toxicological data on pentoxifylline and psammaplin A over a range of concentrations.