Abietadiene synthase catalyzes the committed step in resin acid biosynthesis, forming a mixture of abietadiene double-bond isomers by two sequential, mechanistically distinct cyclizations at separate active sites. The first reaction, protonation-initiated cyclization, converts the universal diterpene precursor geranylgeranyl diphosphate to the stable bicyclic intermediate copalyl diphosphate. In the second, magnesium ion-dependent reaction, diphosphate ester ionization-initiated cyclization generates the tricyclic perhydrophenanthrene-type backbone and is coupled, by intramolecular proton transfer within a transient pimarenyl intermediate, to a 1,2-methyl migration that generates the C13 isopropyl group characteristic of the abietane structure. Alternative deprotonations of the terminal abietenyl carbocation provide a mixture of abietadiene, levopimaradiene, and neoabietadiene, and this product profile varies as a function of pH. Mutational analysis of amino acids at the active site of a modeled structure has identified residues critical for catalysis, as well as several that play roles in specifying product formation, apparently by ligation of a magnesium ion cofactor. These results strongly suggest that choice between alternatives for deprotonation of the abietenyl intermediate depends more on the positioning effects of the carbocation-diphosphate anion reaction partners than on the pKa of multiple participating bases. In one extreme case, mutant N765A is unable to mediate the intramolecular proton transfer and aborts the reaction, without catalyzing 1,2-methyl migration, to produce only sandaracopimaradiene, thereby providing supporting evidence for the corresponding stereochemistry of the cryptic pimarenyl intermediate of the reaction pathway.