The septins are a recently recognized family of proteins that are present in a wide variety of fungal and animal cells, where they are involved in cytokinesis and apparently in other processes involving the organization of the cell surface. Five previously described Saccharomyces cerevisiae septins are associated with the neck filaments of vegetative cells and/or with the developing prospore wall of sporulating cells. We report here the characterization of SPR28, a sixth member of the S. cerevisiae septin gene family whose existence was revealed by the yeast genome project. Analysis of mRNA levels showed that SPR28 is a new member of the group of "late genes' that are expressed at high levels during the meiotic divisions and ascospore formation. The septin it encodes, Spr28p, exhibited specific two-hybrid interactions with itself and with three other septins that are expressed in sporulating cells. Consistent with these results, an Spr28p-green fluorescent protein fusion was induced during meiosis I and appeared to be associated with the developing prospore walls. Deletion of SPR28 in either a wild-type or an spr3 delta background produced no obvious abnormalities in vegetative cells and had little or no effect on sporulation, suggesting that the septins have redundant roles during spore formation.