CAP1, the gene encoding the alpha subunit of Saccharomyces cerevisiae capping protein, was cloned using a probe prepared by PCR with primers based on the amino acid sequence of purified alpha subunit peptides. The sequence is similar to that of capping protein alpha subunits of other species but not to that of the S. cerevisiae capping protein beta subunit or any other protein. Null mutants of capping protein, prepared by deletion of the coding region of CAP1 and CAP2 separately or together, are viable and have a similar phenotype. Deletion of the gene for one subunit leads to a loss of protein for the other subunit. The null mutant has a severe deficit of actin cables and an increased number of actin spots in the mother. Cells are round and relatively large. These features are heterogeneous within a population of cells and vary with genetic background. Overexpression of CAP1 and CAP2 also causes loss of actin cables and cell enlargement, as well as the additional traits of aberrant morphogenesis and cell wall thickening. Capping protein null strains and overexpression strains exhibited normal polarized secretion during bud growth as demonstrated by labeling with fluoresceinated Con A. Projection formation and chitin deposition in response to mating pheromone, mating efficiency, and bud site selection were also normal in capping protein null strains. In addition, bulk secretion of invertase was unimpaired. These data indicate that actin cables are not required for polarized secretion in S. cerevisiae.