The Egr family of transcriptional regulators comprises a group of genes which encode members of the Cys2-His2 class of zinc-finger proteins. We have isolated a zebrafish egr1 homologue by screening a zebrafish genomic library with a mouse Egr1 zinc finger probe. Southern blotting indicated the existence of a single zebrafish egr1 gene and, as in higher vertebrates, the presence of related members of a larger gene family. Sequence analysis of the zebrafish egr1 coding region revealed a high level of homology to the mouse, rat, and human Egr1 genes with the notable exception of a polymorphic, triplet nucleotide repeat sequence in the region coding for the amino terminus of the Egr1 protein. The predicted DNA-binding, zinc-finger domain protein sequence was strictly conserved. The 5' region of the zebrafish egr1 gene contained a variety of transcription factor binding sites, also present in the mouse gene, for serum response factor, CREB and c-Ets. The zebrafish egr1 transcript was approximately 3.4 kb in size and was expressed in adult zebrafish brain and muscle RNA, a pattern of expression similar to that observed in mice. The potential for zebrafish egr1 to function as a transcriptional regulator was tested by constructing an expression vector containing zebrafish egr1 coding sequences under the control of a cytomegalovirus promoter. This construct was found to activate transcription of a reporter plasmid bearing multiple Egr1 binding sites when transiently cotransfected into mouse 3T3 cells. Our results indicate that the structure, regulation, and function of the Egr1 gene have been highly conserved during vertebrate evolution and suggest an important role for this gene in growth and development.