The aromatic amine carcinogen 2-aminofluorene (AF) forms covalent adducts with DNA, predominantly with guanine at the C8 position. Such lesions are bypassed by Y-family polymerases such as Dpo4 via error-free and error-prone mechanisms. We show that Dpo4 catalyzes elongation from a correct 3'-terminal cytosine opposite [AF]G in a nonrepetitive template sequence with low efficiency. This extension leads to cognate full-length product, as well as mis-elongated products containing base mutations and deletions. Crystal structures of the Dpo4 ternary complex, with the 3'-terminal primer cytosine base opposite [AF]G in the anti conformation and with the AF moiety positioned in the major groove, reveal both accurate and misalignment-mediated mutagenic extension pathways. The mutagenic template-primer-dNTP arrangement is promoted by interactions between the polymerase and the bulky lesion rather than by a base pair-stabilized misaligment. Further extension leads to semitargeted mutations via this proposed polymerase-guided mechanism.