We evaluated the roles of the MexAB-OprM efflux pump and beta-lactamase in beta-lactam resistance in Pseudomonas aeruginosa by constructing OprM-deficient, OprM basal level, and OprM fully expressed mutants from beta-lactamase-negative, -inducible, and -overexpressed strains. We conclude that, with the notable exception of imipenem, the MexAB-OprM pump contributes significantly to beta-lactam resistance in both beta-lactamase-negative and beta-lactamase-inducible strains, while the contribution of the MexAB-OprM efflux system is negligible in strains with overexpressed beta-lactamase. Overexpression of the efflux pump alone contributes to the high level of beta-lactam resistance in the absence of beta-lactamase.