A group of seven Sm proteins forms a complex that binds to several RNAs in metazoans. All Sm proteins contain a sequence signature, the Sm domain, also found in two yeast Sm-like proteins associated with the U6 snRNA. We have performed database searches revealing the presence of 16 proteins carrying an Sm domain in the yeast genome. Analysis of this protein family confirmed that seven of its members, encoded by essential genes, are homologues of metazoan Sm proteins. Immunoprecipitation revealed that an evolutionarily related subgroup of seven Sm-like proteins is directly associated with the nuclear U6 and pre-RNase P RNAs. The corresponding genes are essential or required for normal vegetative growth. These proteins appear functionally important to stabilize U6 snRNA. The two last yeast Sm-like proteins were not found associated with RNA, and neither was essential for vegetative growth. To investigate whether U6-associated Sm-like protein function is widespread, we cloned several cDNAs encoding homologous human proteins. Two representative human proteins were shown to associate with U6 snRNA-containing complexes. We also identified archaeal proteins related to Sm and Sm-like proteins. Our results demonstrate that Sm and Sm-like proteins assemble in at least two functionally conserved complexes of deep evolutionary origin.