a number of mutations affecting cAMP phosphodiesterases in Drosophila melanogaster are being studied. Alleles of the dunce locus affect the activity of Form II cAMP-specific phosphodiesterase. Some alleles are null - no detectable Form II activity is observed. Other alleles retain some Form II activity; this activity is being studied to determine if the enzyme is physically different in any way. An altered physical form would be suggestive of dunce being the structural gene for Form II enzyme. A male-sterile mutation, located cytologically in the same chromomere as dunce (chromomere 3D4) appears to affect Form I cAMP phosphodiesterase, an enzyme that also hydrolyzes cGMP. These and other less well characterized mutations are undergoing genetic and biochemical analyses. Both forms of phosphodiesterase are being purified in order to produce antibodies to each form of the enzyme.