Adult-head acetylcholinesterase (AChE) activity from a methyl parathion-resistant strain (OPR) of tobacco budworm, Heliothis virescens (F.), was 27-fold less sensitive to inhibition by methyl paraoxon when compared to AChE activity from a methyl parathion-susceptible strain (OPS). Acetylcholine-sterase activities from adult heads of resistant H. virescens were less sensitive to the N-methyl carbamate propoxur and inhibited by monocrotophos. Acetylcholinesterase activities from a laboratory-maintained, multi-resistant strain (PYR), a standard laboratory-maintained susceptible strain (HRV), and two Georgia field strains (collected from Bainbridge and Tifton) were also assessed. The multi-resistant strain was 27-fold less sensitive to inhibition by methyl paraoxon; AChE activity from the lab-susceptible, HRV strain was comparable to OPS AChE activity; and the field strains were ca. 2-fold less sensitive to inhibition by methyl paraoxon. Responses of AChE to inhibition by monocrotophos (7x10^-5 M) and propoxur (7x10^-4 M) indicated that the OPS and OPR strains were homozygous for the S and R enzyme, respectively. A scatterplot of AChE activities from the PYR, HRV, and the two field strains compared to the OPS and OPR genotypes indicated that the Bainbridge strain was comparable to the OPS strain and possibly homozygous for the S enzyme; whereas, the PYR, HRV, and Tifton strains were not completely homozygous.