In the domain model of eukaryotic genome organization, the functional unit of the genome, along with the relevant regulatory elements, is considered to be a gene or a gene family. In hot-blooded vertebrate animals, the domains of a- and b-globin genes are positioned at different chromosomes and are organized and regulated in different fashion. In cold-blooded animals, in particular in tropical fish Danio rerio, a- and b globin genes are located in a common gene cluster. However, the joint a/b-globin gene cluster is subdivided into two development stage-specific subdomains, the adult one and the embryonic-larval one. In an attempt to find out whether this functional segregation correlates with structural segregation of the domain we compared the DNase I sensitivity and profiles of histone modifications of adult and embryonic-larval segments of the domain in cultured D. rerio fibroblasts. We have demonstrated that, in these nonerythroid cells, adult and embryonic- larval subdomains possess different DNase I sensitivities and different profiles of H3K27me3, a histone modification introduced by PRC2 complex. These observations suggest that joint a/b globin gene domain of Danio rerio is segregated into two structural subdomain harboring adult and embryonic-larval globin genes.