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We identified a complex in S. cerevisiae, the "exosome," consisting of the five essential proteins Rrp4p, Rrp41p, Rrp42p, Rrp43p, and Rrp44p (Dis3p). Remarkably, four of these proteins are homologous to characterized bacterial 3′→5′ exoribonucleases; Rrp44p is homologous to RNase II, while Rrp41p, Rrp42p, and Rrp43p are related to RNase PH. Recombinant Rrp4p, Rrp44p, and Rrp41p are 3′→5′ exoribonucleases in vitro that have distributive, processive, and phosphorolytic activities, respectively. All components of the exosome are required for 3′ processing of the 5.8S rRNA. Human Rrp4p is found in a comparably sized complex, and expression of the hRRP4 gene in yeast complements the rrp4-1 mutation. We conclude that the exosome constitutes a highly conserved eukaryotic RNA processing complex.