The receptors for the platelet-derived growth factor (PDGFRalpha and PDGFRbeta) belong to a subfamily of protein tyrosine kinase receptors that also includes kit and the colony stimulating factor-1 receptor (CSF1R). In mammals, the genes encoding PDGFRalpha and PDGFRbeta are tandemly linked to the kit and CSF1R genes, respectively. Based on the structural similarity and genomic organization of these four genes, it has been suggested that they arose from an ancestral protein tyrosine kinase receptor gene by two rounds of duplication. We have previously cloned the PDGFRbeta and CSF1R genes from the pufferfish, Fugu rubripes, and shown that they are tandemly linked like the mammalian genes [Genome Res. 6 (1996) 1185]. We have now cloned two additional members of this gene family, fPDGFRbeta2 and fCSF1R2 from the fugu and shown that these two genes are also tandemly linked. This indicates that the PDGFRbeta-CSF1R locus has been duplicated in the lineage leading to fugu. The fugu fPDGFRbeta2 and fCSF1R2 genes contain three and one extra introns, respectively, compared with other members of this family. Polymerase chain reaction cloning of a conserved region of PDGFRbeta gene from other ray-finned fishes identified two copies in the zebrafish (order Cypriniformes) and sunfish (order Tetraodontiformes). These results are discussed in the context of the proposed teleost lineage-specific whole genome duplication hypothesis.