Promoter probe fragments containing either the Escherichia coli lacZ (β-galactosidase) gene or the Pseudomonas aeruginosa proC (Δ1-pyrroline 5-carboxylate reductase) gene were fused to the E. coli consensus (tac) promoter and cloned into the broad-host-range vector plasmid pKT240, which also carried the lacIQ repressor gene. Upon induction with isopropyl β-d-thiogalactoside similarly high lacZ and proC enzyme levels were obtained in both E. coli and P. aeruginosa. The E. coli argF promoter, which deviates from the consensus sequence, was clearly less efficient in both hosts. We conclude that the tac/lacIQ system functions well in P. aeruginosa and that no major expression barrier exists between E. coli and P. aeruginosa at the translational level.