Ribonucleotide reductases (RNRs) are a diverse family of enzymes that are alone capable of generating 2'-deoxynucleotides de novo and are thus critical in DNA biosynthesis and repair. The nucleotide reduction reaction in all RNRs requires the generation of a transient active site thiyl radical, and in class I RNRs, this process involves a long-range radical transfer between two subunits, alpha and beta. Because of the transient subunit association, an atomic resolution structure of an active alpha 2 beta 2 RNR complex has been elusive. We used a doubly substituted beta 2, E52Q/(2,3,5)-trifluorotyrosine122-beta 2, to trap wild-type alpha 2 in a long-lived alpha 2 beta 2 complex. We report the structure of this complex by means of cryo-electron microscopy to 3.6-angstrom resolution, allowing for structural visualization of a 32-angstrom-long radical transfer pathway that affords RNR activity.