In West Africa, the cotton bollworm Helicoverpa armigera has recently developed resistance to deltamethrin and cypermethrin. Resistance mechanisms of the strain BK99R9 collected in Bouaké, Ivory Coast in 1999 and selected with deltamethrin were investigated by comparison with a susceptible strain BK77 collected in the same area in 1977. Several approaches were performed: evaluation of the cross-resistance spectrum to various pyrethroids and DDT, effect of a synergist, and by determination of the biochemical characteristics of three enzyme systems (esterases, glutathione-S-transferases, and mixed function oxidases). Deltamethrin resistance in BK99R9 was correlated to an increase of mixed function oxidase. Enhanced monooxygenase levels were then confirmed in several H. armigera field strains collected in cotton areas of West Africa from 1999 to 2001.