Density functional theory calculations using the hybrid functional B3LYP have been performed to study the catalytic mechanism of benzylsuccinate synthase. This enzyme catalyzes the novel addition of the methyl carbon of toluene to fumarate, forming benzylsuccinate and thereby initiating the anaerobic metabolism of toluene in denitrifying bacteria. Benzylsuccinate synthase was suggested to contain a stable glycyl radical, based on sequence similarity to the two known glycyl radical containing enzymes pyruvate-formate lyase and class III anaerobic ribonucleotide reductase. This suggestion was recently confirmed by electron paramagnetic resonance experiments. The calculations demonstrate that an overall homolytic radical mechanism is thermodynamic ally very plausible. The radical is transferred from the stable glycyl radical to toluene via a cysteinyl radical in two hydrogen atom transfer steps, The rate-limitin, step is shown to he the addition of benzyl radical to fumarate, forming a benzylsuccinyl radical intermediate. A full potential energy surface for the benzylsuccinate synthase reactions is presented.