The MPS2 (monopolar spindle two) gene is one of several genes required for the proper execution of spindle pole body (SPB) duplication in the budding yeast Saccharomyces cerevisiae (). We report here that the MPS2 gene encodes an essential 44-kDa protein with two putative coiled-coil regions and a hydrophobic sequence. Although MPS2 is required for normal mitotic growth, some null strains can survive; these survivors exhibit slow growth and abnormal ploidy. The MPS2 protein was tagged with nine copies of the myc epitope, and biochemical fractionation experiments show that it is an integral membrane protein. Visualization of a green fluorescent protein (GFP) Mps2p fusion protein in living cells and indirect immunofluorescence microscopy of 9xmyc-Mps2p revealed a perinuclear localization with one or two brighter foci of staining corresponding to the SPB. Additionally, immunoelectron microscopy shows that GFP-Mps2p localizes to the SPB. Our analysis suggests that Mps2p is required as a component of the SPB for insertion of the nascent SPB into the nuclear envelope.