Twospotted spider mite, Tetranychus urticae Koch, feeds on epidermal cells of foliage, destroys photosynthetic cells, and reduces yield, fiber quality, and seed germination of cotton, Gossypium hirsutum L. With a short life cycle, prolific fecundity, an arrhenotokous reproduction, and an ability to expeditiously digest and detoxify xenobiotics, twospotted spider mite has the propensity to develop resistance to insecticides. Despite mobility, small size, and difficulties associated with handling of twospotted spider mites, this study demonstrated that the 20-ml glass-vial bioassay is a useful technique to evaluate contact toxicity of acaricides against adult mites in a laboratory. A colony of twospotted spider mites was maintained on pinto beans, Phaseolus vulgaris L. in a greenhouse. Abamectin with LC⁵⁰ (95% CL) of 0.014 (0.01–0.02) µg per vial was 1,006 time more toxic than spiromesifen with a LC₅₀ of 14.086 (7.592–42.371) µg per vial. The LC₅₀ values of spiromesifen and propargite were comparable. Bifenazate was 10 times more toxic than dicofol to twospotted spider mite. The order of toxicity of acaricides tested against twospotted spider mite adults was abamectin > bifenazate > dicofol > propargite = spiromesifen. These data are useful for developing baseline contact toxicity for adult twospotted spider mite and monitoring tolerance to acaricides used on cotton in Central Texas.