IN the budding yeast Saccharomyces cerevisiae, the process of conjugation of haploid cells of genotype MAT alpha and MAT alpha to form MAT alpha/alpha diploids is triggered by pheromones produced by each mating type. These pheromones stimulate a cellular response by interaction with receptors linked to a heterotrimeric G protein. Although genetic analysis indicates that the pheromone signal is transmitted through the G beta gamma dimer, the initial target(s) of G protein activation remain to be determined. Temperature-sensitive cells with mutations of the CDC24 and CDC42 genes, which are incapable of budding and of generating cell polarity at the restrictive temperature(1-3), are also unable to mate(4). Cdc24 acts as a guanylyl-nucleotide-exchange factor for the Rho-type GTPase Cdc42(5), which has been shown to be a fundamental component of the molecular machinery controlling morphogenesis in eukaryotic cells(6-10). Therefore, the inability of cdc24 and cdc42 mutants to mate has been presumed to be due to a requirement for generation of cell polarity and related morphogenetic events during conjugation, But here we show that Cdc42 has a direct signalling role in the mating-pheromone response between the G protein and the downstream protein kinase cascade.