Bacterial adaptive immune systems use CRISPRs (clustered regularly interspaced short palindromic repeats) and CRISPR-associated (Cas) proteins for RNA-guided nucleic acid cleavage1,2. Although most prokaryotic adaptive immune systems generally target DNA substrates3,4,5, type III and VI CRISPR systems direct interference complexes against single-stranded RNA substrates6,7,8,9. In type VI systems, the single-subunit C2c2 protein functions as an RNA-guided RNA endonuclease (RNase)9,10. How this enzyme acquires mature CRISPR RNAs (crRNAs) that are essential for immune surveillance and how it carries out crRNA-mediated RNA cleavage remain unclear. Here we show that bacterial C2c2 possesses a unique RNase activity responsible for CRISPR RNA maturation that is distinct from its RNA-activated single-stranded RNA degradation activity. These dual RNase functions are chemically and mechanistically different from each other and from the crRNA-processing behaviour of the evolutionarily unrelated CRISPR enzyme Cpf1 (ref.11). The two RNase activities of C2c2 enable multiplexed processing and loading of guide RNAs that in turn allow sensitive detection of cellular transcripts.