The chemokine receptor CXCR4 and its ligand SDF-1 are essential components of hematopoiesis, organogenesis and immunomodulation in mammalian species. We cloned a cDNA encoding CXCR4 homolog of sterlet (Acipenser ruthenus), a representative of chondrostean fishes. The deduced amino acid sequence of sterlet CXCR4 is almost equally distant from mammalian and teleost CXCR4 (66-68% identical residues). Percent identity with the other chemokine receptors varies in the 30-35% range. The CXCR4 sequences from the three phylogenetically diverged lineages were compared with the sequences of the other chemokine receptors to determine the CXCR4-specific structural elements that were conserved during vertebrate evolution. The characteristic residues and/or motifs are located predominantly in the intracellular and extracellular regions and in the third, fourth and fifth transmembrane domains. The data presented may be helpful for structure-function analysis of the CXCR4 ligand binding and signal transduction.