The Saccharomyces cerevisiae protein Sph1p is both structurally and functionally related to the polarity protein, Spa2p. Sph1p and Spa2p are predicted to share three 100-amino acid domains each exceeding 30% sequence identity, and the amino-terminal domain of each protein contains a direct repeat common to Homo sapiens and Caenorhabditis elegans protein sequences. sph1- and spa2-deleted cells possess defects in mating projection morphology and pseudohyphal growth. sph1(Delta) spa2(Delta) double mutants also exhibit a strong haploid invasive growth defect and an exacerbated mating projection defect relative to either sph1(Delta) or spa2(Delta) single mutants. Consistent with a role in polarized growth, Sph1p localizes to growth sites in a cell cycle-dependent manner: Sph1p concentrates as a cortical patch at the presumptive bud site in unbudded cells, at the tip of small, medium and large buds, and at the bud neck prior to cytokinesis. In pheromone-treated cells, Sph1p localizes to the tip of the mating projection. Proper localization of Sph1p to sites of active growth during budding and mating requires Spa2p. Sph1p interacts in the two-hybrid system with three mitogen-activated protein (MAP) kinase kinases (MAPKKs): Mkk1p and Mkk2p, which function in the cell wall integrity/cell polarization MAP kinase pathway, and Ste7p, which operates in the pheromone and pseudohyphal signaling response pathways. Sph1p also interacts weakly with STE11, the MAPKKK known to activate STE7. Moreover, two-hybrid interactions between SPH1 and STE7 and STE11 occur independently of STE5, a proposed scaffolding protein which interacts with several members of this MAP kinase module. We speculate that Spa2p and Sph1p may function during pseudohyphal and haploid invasive growth to help tether this MAP kinase module to sites of polarized growth. Our results indicate that Spa2p and Sph1p comprise two related proteins important for the control of cell morphogenesis in yeast.