We have cloned and sequenced a gene encoding a yeast homologue of the U1 snRNP 70K protein. The gene, SNP1, encodes a protein which has 30% amino acid identity with the human 70K protein and has a predicted molecular weight of 34 kDa. The yeast and human sequences are more closely related to each other than to other (non-U1) RNA-binding proteins, but diverge considerably in their C-terminal portions. In particular, SNP1 lacks the charged carboxy terminus of the human 70K protein. A yeast strain, a alpha 115, was constructed in which one allele of the SNP1 gene contained a 554 bp deletion. Tetrad analysis of a alpha 115 showed that the SNP1 gene is essential for the viability of yeast cells. The complete human 70K gene did not complement snp1, but the lethal snp1 mutation was rescued by plasmids bearing a chimera in which over half the yeast gene was replaced with the homologous region of the human 70K gene, including the RNA-binding domain. These results suggest that SNP1 encodes a functional homologue of the U1 snRNP 70K protein.