A genomic library of zebrafish was constructed and screened with homeobox-containing probes. One of the positive clones contains a transcribed region which shares extensive sequence homology with the murine Hox-1.4 and Hox-2.6 genes and the human HHO.c13 gene. Characterization of this zebrafish homologue (ZF-13) with respect to expression demonstrated that it is transcribed during embryogenesis where a major RNA species of 2.5 kb and a minor transcript of 4.6 kb are detected. The highest concentration of both transcripts was found in embryos at the stage of somite formation. By in situ hybridization the spatial localization of expression was analysed in hatching embryos. Hybridization signals were mainly detected throughout the neural tube and in the brain. A small amount of RNA derived from ZF-13 was localized in differentiated muscle cells. Our results suggest that homeobox genes of distantly related vertebrate species are very similar with respect to structure and function.