The role of the 5' untranslated region (5'UTR) of the S-layer gene from Thermus thermophilus was analyzed through the isolation of Delta5'UTR mutants. In these mutants the half-life of splA mRNA was strongly reduced and slpA transcription was no longer subjected to growth phase dependent repression. Overproduction and detachment of the external envelopes of the mutants were observed in stationary phase.