We have developed an in vivoexpression technology (IVET) system to identify Actinobacillus pleuroÍpneumoniaegene promoters that are specifically induced in vivoduring infection. This system is based upon an avirulent riboflavin–requiring A. pleuroÍpneumoniaemutant and a promoter-trap vector (pTF86) that contains, in sequence, the T4 terminator, a unique BamHI site, a promoterless copy of the V.harveyi luxABgenes, and a promoterless copy of the B. subtilis ribBAHgenes in the E. coli- A. pleuroÍpneumoniaeshuttle vector pGZRS19. Sau3A fragments of A. pleuroÍpneumoniaegenomic DNA were cloned into the BamHI site in pTF86 and transformed into the A. pleuroÍpneumoniaeRib- mutant. Pigs were infected with pools of 300–600 transformants by endobronchial inoculation and surviving bacteria were isolated from the pigs» lungs at 12–16 h post-infection. Infection strongly selected for transformants containing cloned promoters which drove expression of the vector ribBAHgenes and allowed survival of the Rib- mutant in vivo. Strains that survived in vivo, but which minimally expressed luciferase activity in vitro, should contain cloned promoters that are specifically induced in vivo. Ten clones, designated iviA-J, were isolated which contain promoters that are induced in vivoduring infection. These iviclones were shown to be induced in the animal by luminescence of infected tissue and by direct assay of bacteria recovered from bronchoalveolar lavage. Four of these clones were putatively identified by amino acid sequence similarity as ilvI, the ilvDAoperon, the secE-nusGoperon, and themrpgene. This is the first report of an IVET system for use in the family Pasteurellaceae, as well as the first report of an IVET system utilizing an infection model of pneumonia in the natural host.