We have cloned and sequenced the ERCC2/XPD locus of Xiphophorus maculatus. The human ERCC2/XPD gene is a nucleotide excision repair gene presumed to encode an ATP-dependent DNA helicase. The fish ERCC2/XPD gene is represented on 14.5 kb of genomic DNA and is composed of 23 exons. Within the coding regions, the overall nucleotide identity is 74% compared to the human cDNA. Of 760 amino acids compared between human and fish sequences, 127 differences are observed. Of these differences, 48 residues (38%) represent nonconservative amino acid changes, while 79 (62%) are conservative. The majority (73%) of nonconservative differences between the human and the fish amino acid sequences occur in eight distinct groups comprising only about 10% of the total protein. Overall, the fish and human sequences show 83% amino acid identity and 94% similarity when conservative amino acid substitutions are allowed.