Promoter regions harbouring typical -10 and -35 sites of prokaryotes were isolated from the genome of various species of Frankia using an Escherichia coli promoter-probe plasmid. These promoter regions initiated transcription in E. coli and Streptomyces cells. The consensus promoter sequence for the -10-like site was T A G/A G G/A T and for the -35-like site was T T G T/A C G. Despite the GC pressure observed in Frankia, functionally important positions in the promoter are conserved. As a consequence of these studies, a marker gene having properties compatible with the transcriptional and translational processes of Frankia was built. It is an npt-II gene under the control of three copies of one of the Frankia-E. coli type promoters studied.