Effects of temperature on toxicity of an organophosphate (azinphosmethyl) and several pyrethroids (cypermethrin, fenvalerate, flucythrinate, and permethrin) to Colorado potato beetle, Leptinotarsa decemlineata (Say), were evaluated. Between 14 and 30°C, toxicity of azinphosmethyl increased 3.3-fold while the toxicity of the pyrethroids decreased 3.1- to 8.2-fold. At 35°C, the beetles showed signs of stress and toxicities of the pyrethroids increased, compared with results at 3O°C. These data demonstrate that temperature/toxicity relationships are not only dependent on the chemical group (organophosphate or pyrethroid) but also on temperature range involved and biological responses of the insect. Results suggest that high temperatures may cause control problems in the field, depending on factors such as resistance status, residual effectiveness of the insecticide, behavior of the insect, and effects of fluctuating temperature.