We have isolated a cDNA that encodes a 142 kDa protein by immunoscreening of a Schizosaccharomyces pombe expression library with a new antibody, mAb8, that reveals spindle poles and equatorial ring-like structures in several organisms. This cDNA encodes a putative protein which we termed Alm (for abnormal long morphology). The protein is predicted to be a coiled-coil protein, containing a central alpha-helical domain flanked by non-helical terminal domains. Immunofluorescence analysis showed that Alm1 is localized in the medial region of the cell from anaphase to the end of cytokinesis. Cells carrying an alm1::ura4+ disruption are viable and exhibit an elongated morphology. Homozygous alm1::ura4+ diploids sporulated normally but the spores did not germinate. Spores that have inherited the disruption allele from a heterozygous alm1+/ alm1::ura4+ diploid germinated but generated smaller colonies. We propose that Alm1 participates in the structural organization of the medial region in S. pombe.