The insect repellents 3535 (ethyl butylacetylaminopropionate or IR3535) and deet (MN-diethyl-3-methylbenzamide) were prepared as 2OVa solutions in absolute ethanol and evaluated fbr repellency againstmany mosquito species in Thailand under laboratory and field conditions using human subjects. In the laboratory,0.1 ml was applied per 3O-cm' of exposed area on a volunteer's forearm (0.66-0.67 mg active ingredient [AI]/cm2), whereas in the field, volunteers' legs (from knee to ankle, with a surface area of about 712-782 cm2) weretreated with 3 ml per exposed area (0.76-0.84 mg Al/cm' ). In the laboratory, both IR3535 and deet showedequal repellency (P > 0.05) for 9.8 and 9.7 h against Aedes aegypti, for 13.1 and 12.1 h against Culex quinquefasciatus,and for 14.8 and 14.5 h against Cx. tritaeniorhynchus, respectively. Anopheles dirzs was significantlyless sensitive to IR353-5 than to deet (P < 0.05), with a mean protection time of 3.8 and 5.8 h, respectively.Under field conditions, both IR3535 and deet provided a high degree of protection against various mosquitovectors ranging from 94 to looa/a during the test periods. Both repellents provided a high level of protection forat least 8 h against Ae. albopictus and for at least 5 h against Cr. gelidus, Cx. tritaeniorhynchus, Cx. quinquefasciatus,Mansonia dives, Ma. uniformis, Ma- annulata, Ma. annuliftra, Anopheles minimus, andAn. maculatus.This study clearly documents the potential of IR3535 for use as a topical treatment against a wide range ofmosquito species belonging to several genera