The ability to switch between yeast and hyphal morphologies is an important virulence factor for the opportunistic pathogenCandida albicans. Although the kinetics of appearance of the filamentous ring that forms at the incipient septum differ in yeast and cells forming hyphae (germ tubes) ( Soll and Mitchell, 1983 ), the molecular mechanisms that regulate this difference are not known. Int1p, a C. albicans gene product with similarity in its C terminus to Saccharomyces cerevisiae Bud4p, has a role in hyphal morphogenesis. Here we report that in S. cerevisiae, Int1p expression results in the growth of highly polarized cells with delocalized chitin and defects in cytokinesis and bud-site selection patterns, phenotypes that are also seen in S. cerevisiae septin mutant strains. Expression of high levels of Int1p in S. cerevisiae generated elaborate spiral-like structures at the periphery of the polarized cells that contained septins and Int1p. In addition, Int1p coimmunoprecipitated with the Cdc11p and Cdc12p septins, and Cdc12p is required for the establishment and maintenance of these Int1p/septin spirals. Although Swe1p kinase contributes to INT1-induced filamentous growth inS. cerevisiae, it is not required for the formation of ectopic Int1p/septin structures. In C. albicans, Int1p was important for the axial budding pattern and colocalized with Cdc3p septin in a ring at the mother-bud neck of yeast and pseudohyphal cells. Under conditions that induce hyphae, both Cdc3p and Int1p localized to a ring distal to the junction of the mother cell and germ tube. Thus, placement of the Int1p/septin ring with respect to the mother–daughter cell junction distinguishes yeast/pseudohyphal growth from hyphal growth in C. albicans.