Helicoverpa armigera is a key-pest of cotton in West Africa, controlled by pyrethroids for more than twenty years. In the Côte d'Ivoire, susceptibility to pyrethroids is monitored every year on populations collected in Bouaké at the end of the season (BK strains). A loss of effectiveness to cypermethrin and deltamethrin has been observed for the last three years. Compared to the mean of 1985-94 LD 50 values of the BK strains and to the LD50 of a susceptible laboratory strain (BK77), the H. armigera population collected in 1997 (BK97) expresses factors of resistance varying from 9 to 27 for deltamethrin. Using PBO, a threefold reduction in the LD 50 is obtained in the resistant strain (BK97) while DEF has no effect. Observing that the LD 50 obtained with PBO on the resistant grain remains tenfold above the susceptible one, we conclude that a) MFO are involved in the pyrethroid resistance mechanism and b) another one, excluding esterases, is probably occurring in this field population. The synergistic effect of PBO bas been confirmed on another H. armigera population (NBG97) collected on the Burkina border.