G protein-mediated signal transduction systems have been identified in a diverse group of eukaryotic organisms, including yeast, plants, Dictyostelium and animals. G protein signaling components have been identified in many of these organisms, from the seven transmembrane domain receptors to distinct alpha, beta and gamma subunits of the heterotrimeric G protein and the intracellular effectors which they regulate. Their broad distribution and sequence conservation implies that genes encoding the components of G protein signaling evolved with early eukaryotes. Their subsequent proliferation among eukaryotic organisms provides an opportunity to study the coevolution of these interacting multigene families. We have focused our interests on G protein alpha subunits, which bind and hydrolyze GTP and interact with receptors and effectors. Gene structure and nucleotide sequence comparisons provided a comprehensive picture of G alpha evolution. Sequence comparisons identified three major groups of G alpha genes, termed the GPA, the G alpha-I and G alpha-II Groups. G alpha genes within the three Groups have evolved at different rates. The GPA Group is primarily composed of G alpha genes from fungi, plants, and slime mold. Within the G alpha-I and G alpha-II Groups, four classes of genes have been identified based upon sequence comparisons and functional similarities; Gi, Gq, G12, and GS. Members of all four classes are expressed in invertebrates and vertebrates but not in other eukaryotes, suggesting that this quartet evolved with metazoan progenitors.