A field trial was carried out on Lambarene Island, Gabon, against Mansonia spp. With ultra low volume applications of Reslin 15S, a synthetic pyrethroid formulation containing 2.0% w/v S-bioallethrin ((+)-transchrysanthemic ester of (+)-allethrolone) and 13.0% w/v bioresmethrin ((+)_trans chrysanthemic ester of 5-benzyl-3-furyl-methanol) applied with a Leco HD cold aerosol ULV generator. The level of control was determined by bait catches at fixed sites. The treatments gave very high immediate kill of adult mosquitoes and, although penetration into the main breeding sites of the ULV mist was hampered by physical conditions, the level of control obtained during the trial was better than expected from a Mansonia spp. population model incorporating a similar spraying regime.