The Saccharomyces cerevisiae mnn10 mutant is defective in the synthesis of N-linked oligosaccharides (Ballou et al., 1989). This mutation has no effect on O-linked sugars, but results in the accumulation of glycoproteins that contain severely truncated N-linked outer-chain oligosaccharides. We have cloned the MNN10 gene by complementation of the hygromycin B sensitivity conferred by the mutant phenotype. Sequence analysis predicts that Mnn10p is a 46.7 kDa type II membrane protein with structural features characteristic of a glycosyltransferase. Subcellular fractionation data indicate that most of the Mnn10 protein cofractionates with Golgi markers and away from markers for the endoplasmic reticulum (ER), suggesting Mnn10p is localized to the Golgi complex. A comparison of the Mnn10 protein sequence to proteins in the two different databases identified five proteins that are homologous to Mnn10p, including a well characterized Schizosaccharomyces pombe alpha 1,2 galactosyltransferase that resides in the Golgi complex. Taken together, these results suggest that MNN10 encodes a novel Golgi-localized mannosyltransferase contained in this previously unrecognized family of related sugar transferases.