LC(50) values of adult Heliothis virescens (F.) males were obtained by the vial technique in 1988 and 1989 and compared to field performance of cypermethrin applications to replicated cotton fields at 0.06, 0.07 and 0.08 lb ai/A and to untreated check fields. LC(50) values indicate a wide range of variability in response to cypermethrin treated vials at sampling dates throughout both years. Significant changes in LC(50) from one sample date to another were few and did not always correspond to significant changes in damaged squares at any of the rates tested. High levels of commercially acceptable control (>94%) were obtained with all use rates of cypermethrin regardless of within season changes in LC(50) obtained from adult vial tests.