It is well understood how mobile introns home to allelic sites, but how they are stimulated to transpose to ectopic locations on an evolutionary timescale is unclear. Here we show that a group I intron can move to degenerate sites under oxidizing conditions. The phage T4 td intron endonuclease, I-TevI, is responsible for this infidelity. We demonstrate that I-TevI, which promotes mobility and is subject to autorepression and translational control, is also regulated posttranslationally by a redox mechanism. Redox regulation is exercised by a zinc finger (ZF) in a linker that connects the catalytic domain of I-TevI to the DNA binding domain. Four cysteines coordinate Zn(2+) in the ZF, which ensures that I-TevI cleaves its DNA substrate at a fixed distance, 23-25 nucleotides upstream of the intron insertion site. We show that the fidelity of I-TevI cleavage is controlled by redox-responsive Zn(2+) cycling. When the ZF is mutated, or after exposure of the wild-type I-TevI to H(2)O(2), intron homing to degenerate sites is increased, likely because of indiscriminate DNA cleavage. These results suggest a mechanism for rapid intron dispersal, joining recent descriptions of the activation of biomolecular processes by oxidative stress through cysteine chemistry.