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Graphical abstract Bioassays with synergists can provide a quick and easy basis for initial characterization of resistant mosquito populations, without the need of preserved specimens, expensive equipment and substrates or specialized expertise. Pictograme: Knock down time responses of a resistant Anopheles gambiae population from Tiassalé to deltamethrin. A significant reduction in knockdown time was observed after 20 min pre-exposure to the P450 inhibitor (PBO). KDT50 shifted from 63.32 min for deltamethrin alone to 21.86 min for deltamethrin + PBO., Highlights • Resistance to deltamethrin was confirmed in the mosquito population from Tiassale, Ivory Coast. • Pre-exposure to esterase synergists yielded an increase mortality and knockdown. • Pre-exposure to oxidase synergists yielded an increase mortality and knockdown. • Pre-exposure to a P-gps inhibitor caused no significant increase in mortality and knockdown. • Use of synergists is a convenient method for investigating potential metabolic resistance mechanisms in field mosquitoes., Metabolic resistance and the potential role of permeability-glycoprotein (P-gp) efflux pumps were investigated in a pyrethroid-resistant wild Anopheles gambiae s.l. Tiassalé population, using WHO susceptibility assays with deltamethrin (0.05%), with and without pre-exposure to synergists. The synergists used included an inhibitor of P-glycoprotein efflux pumps (verapamil), an inhibitor of esterases (EN 16-5), and an inhibitor of P450s and esterases (piperonyl butoxide). Pre-exposure to verapamil followed by deltamethrin led to a slight but non-significant (P = 0.59) increase in mortality relative to exposure to deltamethrin alone (64.5% versus 69.2%). Similarly, pre-exposure to EN 16-5 yielded a non-significant increase in mortality (to 76.6%; P = 0.85) but a significant increase in the knock down rate (from 48.3% to 78.7%; P