The yeast alpha-factor pheromone receptor (Ste2) belongs to the large superfamily of G protein-coupled receptors (GPCRs) that characteristically contain seven transmembrane domains (TMs). A wide range of GPCRs are thought to exist as dimers or oligomers. To identify the interface regions that mediate oligomerization of Ste2, a set of 73 different mutants with Cys residues substituted near the extracellular ends of the transmembrane domains were screened for the ability to form intermolecular disulfide bonds. Disulfide bonds formed between Cys residues at six positions in Ste2. Cys substituted for Val-45 formed disulfide bonds, indicating contact between residues at the extracellular end of TM1. Disulfide bonds also formed with Cys residues substituted for five different residues clustered near the extracellular end of TM4 (Val-183, Val-186, Lys-187, Met-189, and Ile-190). Binding of the alpha-factor ligand to Ste2 did not change the sites at which cross-linking occurred in these TMs, but it did increase the efficiency of dimer formation for the Ste2-V183C mutant. Interestingly, oligomers of the class A family of vertebrate GPCRs are also thought to form homomeric contacts at TM1 and TM4. These results support the conclusion that GPCRs form oligomers and not just dimers, since TM1 and TM4 are too far apart in the class A GPCRs to form contacts in the same dimer moiety. Similar dimer interface sites in Ste2 and class A receptors provide further evidence that many aspects of structure and function are highly conserved across the divergent GPCR superfamily.