A high level of resistance to d-trans permethrin [3-Phenoxybenzyl-d-trans-3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylate] of >;4000-fold has been induced in larvae of the southern house mosquito by selection of a California population in the laboratory. Cross-resistance involves d-cis permethrin [3-phenoxybenzyl-d-cis-3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylate] at 1021-fold and various other pyrethroids. Resistance is relatively high in heterozygotes (ca. 40-fold). The magnitude of resistance and the limited effect of the synergists, piperonyl butoxide and DEF® (S,S,S-tributyl phosphorotrithioate), on the resistant strain suggest that non-metabolic mechanisms such as reduced sensitivity of the target site may be the primary source of resistance.