The dunce locus of Drosophila melanogaster is considered to house a gene involved in memory, because flies carrying lesions at the locus have shortened memory of several different conditioned behaviours. Our recent partial characterization of the gene at the molecular level, along with prior genetic and biochemical evidence, recently provides compelling evidence that the gene codes for the enzyme cAMP phosphodiesterase. The observation that the gene encodes at least six overlapping poly(A)+ RNA molecules ranging in size from 4.2 to 9.5 kilobases (kb) (ref. 8), suggests that the gene is extraordinarily complex. Here we provide the sequence of a dunce complementary DNA clone and the corresponding genomic coding regions which show that the organization of the gene is elaborate. The cDNA clone defines dunce exons which are separated by a large intron of 79 kb. More importantly, at least two other genes are shown to reside within the large intron, including the well-defined glue protein gene, Sgs-4. The location of dunce exons relative to the molecular breakpoints of chromosomal aberrations with defined cytological positions indicates that the dunce gene extends over more than five polytene chromosome bands.