Data are presented on the partial purification and properties of a 5 S ribosomal RNA maturation nuclease, termed RNase M5, from Bacillus subtillis 168. RNase M5 specifically cleaves 21 and 42 nucleotides, respectively, from the 5' and 3' termini of a 5 S rRNA precursor to yield the mature (116 nucleotides) 5 S rRNA. The cleavage is endonucleolytic with the formation of 5'-phosphoryl and 3'-hydroxyl groups. Enzyme action requires divalent cations, which may be furnished by either certain metals or by polyamines. The activity is separable into two components both of which are required for activity. It appears that the same nuclease excises the 5'- and 3'-terminal segments since preparations lose the capacity to modify the two termini with an identical first order thermal decay rate. Certain features of the rRNA precursor which may be involved in cognitive interaction with RNase M5 are discussed.