The yeast-specific alpha 1,2-mannosidase, Mns1p, converts Man,GlcNAc2 to a single isomer of Man8GlcNAc2 during N-linked oligosaccharide processing in Saccharomyces cerevisiae. Mns1p is a 68 kDa type II integral membrane glycoprotein with a very short amino terminal cytoplasmic tail of only two amino acids and a large carboxy-terminal catalytic region that is homologous to class 1 alpha 1,2-mannosidases from mammalian and other species. We have used immunofluorescence and immunoelectron microscopy to demonstrate that Mns1p is localized in the endoplasmic reticulum in Saccharomyces cerevisiae. As Mns1p contains none of the known endoplasmic reticulum retrieval motifs (HDEL, KK or RR), these results suggest that Mns1p is localized in the endoplasmic reticulum by a different retentin mechanism.