Growth rate-dependent expression of the rnpB gene encoding M1 RNA, the RNA component of Escherichia coli RNase P, was examined using a hybrid transducing λ phage was constructed carrying the lacZ gene fused to the promoter for the rnpB gene. The amounts of β-galactosidase were analyzed in the E. coli lysogen carrying this transducing phage grown in different growth media to see dependence of the rnpB transcription on growth rate. The synthesis of β-galactosidase from the rnpB-lacZ fusion increased with growth rate. This result suggests that the rnpB promoter activity is dependent on the growth rate.