We have constructed a derivative of the broad-host-range vector RSF1010. This plasmid, pαΩ, contains an expression cassette derived from bacteriophage T4 gene 32, into which we have inserted the coding sequence for the xylE enzyme (C2,30) of the TOL plasmid pWWO. The composite plasmid, pαxylEΩ, was transferred by conjugal mobilisation into a variety of Gram-negative bacteria (Agrobacter, Paracoccus, Erwinia, Pseudomonas, Rhizobium and Xanthomonas). High levels of C2,30 activity were found in almost all of the extracts. Polyacrylamide gel electrophoresis of these extracts revealed a prominent protein band at 35 kDa whose identity as the C2,30 gene product was confirmed by immunoblotting. We have mapped the 5′ ends of the gene 32/xylE hybrid transcripts. In all of the Gram-negative bacteria, the proximal P2 promoter is the most efficient promoter in the cassette. In most of the strains a weaker and more distal promoter activity (P1) was also detected. In both uninfected and phage-infected Escherichia coli cells, the transcript produced from this promoter is processed at a specific site upstream from the gene 32 start codon. The same processing occurred in all the bacterial species examined. The decay of the hybrid xylE transcript has been analyzed in E. coli and Erwinia, and in both strains this mRNA was among the most stable.