The steady-state catalytic mechanism of a fatty acid alpha-(di)oxygenase is examined, revealing that a persistent tyrosyl radical (Tyr379*) effects O-2 insertion into C-alpha-H bonds of fatty acids. The initiating C-alpha-H homolysis step is characterized by apparent rate constants and deuterium kinetic isotope effects (KIEs) that increase hyperbolically upon raising the concentration of O-2. These results are consistent with H* tunneling, transitioning from a reversible to an irreversible regime. The limiting deuterium KIEs increase from similar to 30 to 120 as the fatty acid chain is shortened from that of the native substrate. In addition, activation barriers increase in a manner that reflects decreased fatty acid binding affinities. Anaerobic isotope exchange experiments provide compelling evidence that Tyr379* initiates catalysis by H* abstraction. C-alpha-H homolysis is kinetically driven by O-2 trapping of the alpha-carbon radical and reduction of a putative peroxyl radical intermediate to a 2(R)-hydroperoxide product. These findings add to a body of work which establishes large-scale hydrogen tunneling in proteins. This particular example is novel because it involves a protein-derived amino acid radical.