Serratia marcescens encodes an inducible, chromosomal beta-lactamase, ampC. Studies addressing the regulation of inducible ampC genes have focused primarily on Enterobacter cloacae and Citrobacter freundii. The purpose of this study was to clone and sequence the ampC, ampR and intergenic region of S. marcescens and examine both inducible and basal level ampC expression. Sequence analysis of the S. marcescens ampC gene identified an extended 5' untranslated region (UTR) of 126 nucleotides, which formed a prominent stem-loop structure. Induction of ampC expression required AmpR, and the start of transcription was determined using primer extension analysis. In vivo half-life analysis revealed that the half-life of the S. marcescens ampC transcript was 7 min. Confirmation of the in vivo half-life and the role of the stem-loop structure in the 5' UTR was demonstrated by comparing transcript half-life and luciferase expression between a wild-type (WT) and a 5' UTR stem-loop deletion mutant. These data demonstrated that the stem-loop structure was involved in transcript stability. Taken together, these findings indicate that constitutive expression of S. marcescens ampC is regulated by both transcriptional initiation and post-transcriptional events.