The Huntington's disease (HD) gene contains a trinucleotide repeat that is expanded and unstable in patients with the disease (HDCRG, 1993). As the first step toward investigating a potential role for this gene in early vertebrate development, we isolated the homolog of the Huntington's disease (ZHD) cDNA in zebrafish. This cDNA encodes a predicted protein product of 3121 amino acids with 70% identity to human huntingtin. The first exon is predicted to encode four glutamines, followed by only one proline, demonstrating that the polymorphic polyproline stretch found in mammalian HD sequences is absent in the fish. We sequenced approximately 900 bp upstream from the predicted start codon and found that it lacks a TATA box, CCAAT box, or Sp1 binding sites. Western blot analysis revealed that the protein is expressed at a high level in late embryonic development and at moderate levels in the adult head.