cDNA encoding a novel phosphodiesterase (PDE) was isolated from a human fetal lung cDNA library and designated PDE10A. The deduced amino acid sequence contains 779 amino acids, including a putative cGMP binding sequence in the amino-terminal portion of the molecule and a catalytic domain that is 16–47% identical in amino acid sequence to those of other PDE families. Recombinant PDE10A transfected and expressed in COS-7 cells hydrolyzed cAMP and cGMP with K m values of 0.26 and 7.2 μ m , respectively, and V max with cGMP was almost twice that with cAMP. Of the PDE inhibitors tested, dipyridamole was most effective, with IC 50 values of 1.2 and 0.45 μ m for inhibition of cAMP and cGMP hydrolysis, respectively. cGMP inhibited hydrolysis of cAMP, and cAMP inhibited cGMP hydrolysis with IC 50 values of 14 and 0.39 μ m , respectively. Thus, PDE10A exhibited properties of a cAMP PDE and a cAMP-inhibited cGMP PDE. PDE10A transcripts were particularly abundant in the putamen and caudate nucleus regions of brain and in thyroid and testis, and in much lower amounts in other tissues. The PDE10A gene was located on chromosome 6q26 by fluorescent in situ hybridization analysis. PDE10A represents a new member of the PDE superfamily, exhibiting unique kinetic properties and inhibitor sensitivity.