Through the use of somatic cell hybrids segregating either human or rat chromosomes, we determined the chromosome localizations of two genes encoding cAMP-specific phosphodiesterases (cAMP-PDEs). PDE4D, the gene encoding the cAMP-PDE isoform 3 (IVd), was assigned to human chromosome 5 and rat chromosome 2, and PDE4B, the gene encoding the cAMP-PDE isoform 4 (IVb), was assigned to human chromosome 1 and rat chromosome 5. These localizations extend the homology between rat chromosome 2 and human chromosome 5, on the one hand, and between rat chromosome 5 and human chromosome 1, on the other hand.