Upon exposure to mating pheromone, Saccharomyces cerevisiae undergoes cellular differentiation to form a morphologically distinct cell called a "shmoo". Double staining experiments revealed that both the SPA2 protein and actin localize to the shmoo tip which is the site of polarized cell growth. Actin concentrates as spots throughout the shmoo projection, while SPA2 localizes as a sharp patch at the shmoo tip. DNA sequence analysis of the SPA2 gene revealed an open reading frame 1,466 codons in length; the predicted protein sequence contains many internal repeats including a nine amino acid sequence that is imperfectly repeated 25 times. Portions of the SPA2 sequence exhibit a low-level similarity to proteins containing coiled-coil structures. Yeast cells containing a large deletion of the SPA2 gene are similar in growth rate to wild-type cells. However, spa2 mutant cells are impaired in their ability to form shmoos upon exposure to mating pheromone, and they do not mate efficiently with other spa2 mutant cells. Thus, we suggest that the SPA2 protein plays a critical role in cellular morphogenesis during mating, perhaps as a cytoskeletal protein.