The Saccharomyces cerevisiae alg3-1 mutant is descilbed as defective in the biosynthesis of dolichol-linked oligosaccharides (Huffaker and Robbins, Proc. Natl. Acad. Sci. USA, 80, 7466–7470, 1983). Man₅GlcNAc₂-PP-Dol accumulates in alg3 cells and Endo H resistant carbohydrates are transferred to protein by the oligosaccharyltransferase complex. In this study, we describe the cloning of the ALG3 locus by complementation of the temperature sensitive growth defect of the alg3 stt3 double mutant. The isolated ALG3 gene complements both the defect in the biosynthesis of lipidlinked oligosaccharides of the alg3-mutant and the underglycosylation of secretory proteins. The inactivation of the nonessential ALG3 gene results in the accumulation of lipid-linked Man₅GlcNAc₂ and protein-bound carbohydrates which are completely Endo H resistant. The ALG3 locus encodes a potential ER-transmembrane protein of 458 amino acids (53 kDa) with a C-terminal KKXX-retrieval sequence.