Mammalian mitochondrial genome encodes a small set of tRNAs,rRNAs,and mRNAs.The RNA synthesis process has been well characterized.How the RNAs are degraded,however,is poorly understood.It was long assumed that the degradation happens in the matrix where transcription and translation machineries reside.Here we show that contrary to the assumption,mammalian mitochondrial RNA degradation occurs in the mitochondrial intermembrane space (IMS) and the IMS-localized RNASET2 is the enzyme that degrades the RNAs.This provides a new paradigm for understanding mitochondrial RNA metabolism and transport.