Functional ribosomal RNAs are generated from longer precursor species in every organism known. Maturation of the 5' side of 16S rRNA in Escherichia coli is catalysed in a two-step process by the cooperative action of RNase E and RNase G. However, many bacteria lack RNase E and RNase G orthologues, raising the question as to how 16S rRNA processing occurs in these organisms. Here we show that the maturation of Bacillus subtilis 16S rRNA is also a two-step process and that the enzyme responsible for the generation of the mature 5' end is the widely distributed essential ribonuclease YkqC/RNase J1. Depletion of B. subtilis of RNase J1 results in an accumulation of 16S rRNA precursors in vivo. The precursor species are found in polysomes suggesting that they can function in translation. Mutation of the predicted catalytic site of RNase J1 abolishes both 16S rRNA processing and cell viability. Finally, purified RNase J1 can correctly mature precursor 16S rRNA assembled in 70S ribosomes, showing that its role is direct.