The concept of using wide-mesh jackets treated with a synthetic pyrethroid rather than a conventional vapor repellent for protection against biting flies is examined. This principle, previously studied by the USDA Insects Affecting Man and Animals Research Laboratory at Gainesville, Florida, is based on the hypothesis that the toxic action of the treatment would quickly affect biting behavior as well as reduce insect populations in the vicinity of subjects wearing jackets to levels where effective protection is provided. Jackets treated with (a) 3-phenoxybenzyl (+) Cis, trans-2, 2, - dimethyl-3 - (dichlorovinyl) cyclopropane carboxylate (permethrin), (b) the standard repellent N, N - diethyl -m- toluamide (Deet) and (c) the experimental repellent tetrahydrofurfuryl octanoate were compared for their efficacy against black flies and mosquitoes in a series of field tests at Canadian Forces Base (CFB) Petawawa in Ontario. The ability of repellent-treated hoods to provide protection for the wearer's face was also investigated. The jackets treated with permethrin provided subjects with good protection against black flies and mosquitoes but, based on averaged landing counts, were not as effective as jackets treated with Deet, especially for protecting the face during initial exposure to the insects. The toxic action of permethrin-treated jackets reduced the black fly and mosquito population in a given area to much lower levels after approximately 10 min exposure after which effective protection was afforded. Hoods treated with Deet were as effective as jackets treated with Deet or with tetrahydrofurfuryl octanoate in providing protection for the face. Against the black fly and mosquito species active during testing, tetrahydrofurfuryl octanoate was found to be as effective as Deet when used as a jacket treatment.