A deletion derivative of the ermC gene was constructed that expresses a 254-nucleotide mRNA. The small size of this mRNA facilitated the detection of processing products that did not differ greatly in size from the full-length transcript. In the presence of erythromycin, which induces ribosome stalling near the 5' end of ermC mRNA, the 254-nucleotide mRNA was cleaved endonucleolytically at the site of ribosome stalling. Only the downstream product of this cleavage was detectable; the upstream product was apparently too unstable to be detected. The downstream cleavage product accumulated at times after rifampicin addition, suggesting that the stalled ribosome at the 5' end conferred stability to this RNA fragment. Neither Bs-RNase III nor RNase M5, the two known narrow-specificity endoribonucleases of Bacillus subtilis, was responsible for this cleavage. These results indicate the presence in B. subtilis of another specific endoribonuclease, which may be ribosome associated.