Resistance to pyrethroid insecticides was diagnosed in Heliothis virescens from cotton fields in Texas in 1985 by Plap and Campanhola. In 1986, control failures with pyrethroid insecticides were reported at several sites in Mississippi. Topical assays in the laboratory showed 5-23 fold resistance levels in larvae obtained from three of these sites. Bioassays of susceptible third instar larvae on cotton terminal buds sprayed with recommended rates of pyrethroids showed 90-95% mortality, but gave only 40-57% mortality of the field-collected resistant larvae. There was a close correlation between the frequency of resistant phenotypes on the spray tables and in an adult assay technique. The resistance appears to be incompletely recessive in inheritance. On the basis of these results, careful use of pyrethroid insecticides to reserve their usefulness is recommended.