In a laboratory, resistance tests were conducted with larvae of the salt-marsh sand fly Culicoides furens (Poey) from 2 Florida marshes, 1 of which had never been treated for sand fly control and the other had been treated 3 times with dieldrin, once with malathion, and once with heptachlor, each at 1 pound per acre, over a 4-year period. Larvae from the treated marsh showed greater than 100-fold resistance to dieldrin, heptachlor, chlordane, and lindane, about 10-fold resistance to endrin, and no resistance to DDT, malathion, parathion, and Bayer 21/199.