Formulations of the repellent diethyl methyl benzamide (deet) were evaluated against mosquitoes in field tests conducted in Sisaket Province, northeastern Thailand. Thirty-three percent deet in an applicator stick provided > or = 87.1% protection against primarily Culex vishnui, Cx. gelidus, and Cx. tritaeniorhynchus for up to 5 h, and 50% deet in ethanol provided > or = 93.3% protection against Anopheles sp. and culicines for up to 8 h. The U.S. Army Extended Duration Repellent Formulation ([EDRF], containing 33% deet), 50% deet in ethanol, and 75% deet in ethanol provided complete protection for up to 6 h against primarily Culex vishnui and Anopheles dirus.