Five new synthetic pyrethroids were evaluated against 6 species of mosquitoes in the laboratory. They were very effective against 4th instar larvae of the various species having an LC90 in the range of less than 1 ppb for the most active compound to 50 ppb for the least active. Most of the pyrethroids were highly effective against pupae, having an LC90 in the range of 1-10 ppb. Some of the compounds not previously evaluated were studied in the field against stagnant and flood-water mosquitoes. Permethrin and Sumithrin [d-(cis, trans) phenothrin] were effective in the range of 0.025 Ib/A against OP resistant Aedes nigromaculis, with the former being slightly more active. Against stagnant-water mosquitoes, both Sumithrin and Sumicidin (a-cyano-m-phenoxybenzly a-isopropyl-p-chlorophenyl-acetate) proved effective in the range of 0.025 Ib/A, the latter being somewhat more active. In general both compounds controlled mosquito larvae for more than 7 days with 1 treatment.