The mating pheromone response in S. cerevisiae is activated by a G protein-mediated signaling pathway in which G beta gamma is the active transducer of the signal. When exogenous pheromone is added to vegetatively growing cells, G beta is rapidly phosphorylated at several sites; phosphorylation does not require de novo protein synthesis. A mutation in G beta was constructed that eliminates signal-induced phosphorylation. This mutation leads to enhanced sensitivity to and impaired ability to recover from pheromone, but does not affect the ability of G beta gamma to transmit the mating signal. These phenotypes suggest that G protein phosphorylation mediates an adaptive response to pheromone-induced signaling. G beta phosphorylation does not require either the pheromone receptor C-terminus or the product of the SST2 gene, both of which mediate separate adaptive responses to pheromone. However, G beta phosphorylation is greatly facilitated by the presence of the G alpha subunit, which has also been shown to participate in an adaptation to pheromone.