Translocation of proteins across the endoplasmic reticulum (ER) membrane represents the first step in the eukaryotic secretory pathway. In mammalian cells, the targeting of secretory and membrane protein precursors to the ER is mediated by signal recognition particle (SRP), a cytosolic ribonucleoprotein complex comprising a molecule of 7SL RNA and six polypeptide subunits (relative molecular masses 9, 14, 19, 54, 68 and 72K). In Saccharomyces cerevisiae, a homologue of the 54K subunit (SRP54) co-purifies with a small cytoplasmic RNA, scR1 (refs 4, 5). Genetic data indicate that SRP54 and scR1 are involved in translocation in vivo, suggesting the existence of an SRP-like activity in yeast. Whether this activity requires additional components similar to those found in mammalian SRP is not known. We have recently reported a genetic selection that led to the isolation of a yeast mutant, sec65-1, which is conditionally defective in the insertion of integral membrane proteins into the ER. Here we report the cloning and sequencing of the SEC65 gene, which encodes a 31.2K protein with significant sequence similarity to the 19K subunit of human SRP (SRP19). We also report the cloning of a multicopy suppressor of sec65-1, and its identification as the previously defined SRP54 gene, providing genetic evidence for an interaction between these gene products in vivo.