Adults and larvae of western flower thrips, Frankliniella occidentalis (Pergande), were collected from cotton in a greehouse during the winter and the field in 2006. A glass vial bioassay was developed to determine dosage mortality data for dicrotophos, methamidophos, and spinosad. Based on LC50 values, toxicity of spinosad was signigicantly higher than either dicrotophos or methamidophos. This baseline toxicity data should be important in insecticide selection and evaluation of efficacy of the continued use of these insecticides for thrips control on cotton.