The transcription initiation site of the xylDEGF operon on the TOL plasmid of Pseudomonas putida mt-2 was determined in P. putida and in Escherichia coli by S1 nuclease and reverse transcriptase mapping. The induced synthesis of mRNA started at the same start point in both P. putida and E. coli, although the amount of mRNA in E. coli cells was less than that in P. putida. The nucleotide sequence of the region surrounding the start point was also determined. The ribosome-binding site (RBS) complementary to the 3' end of the 16S rRNA of Pseudomonas aeruginosa and E. coli preceded the predicted start codon for the xylD gene. The consensus nucleotide sequence for E. coli promoters was not found in the region preceding the transcription start point. On the other hand, the sequences of the "-10" and the "-35" regions of the xylDEGF operon revealed some homology with the respective, previously determined sequences of the xylABC operon of the TOL plasmid.