ABSTRACT Chromomere 3D4 of the X chromosome of D. melanogaster contains two genes, dunce (dnc) and sperm amotile (sam). Mutations in dnc cause defects in memory formation and female fertility and reduce or eliminate the activity of a cAMP-specific phosphodiesterase designated form II. A fine structure map of this region has been constructed showing the locations of two sam mutations, five dnc mutations and a newly identified locus designated control of fertility (cf) that acts in cis to regulate the female sterility phenotype of dnc. The two sam mutations are separated by 0.02 ± 0.01 cM, the rightmost being located 0.08 ± 0.02 cM to the left of the null mutation dncm 11. A cluster of null and form II-defective dnc mutations is located 0.04 ± 0.01 cM to the right of dncm 11. The cf locus is 0.06 ± 0.02 cM to the right of this cluster. The location of the dnc and cf sites identify a region of approximately 0.10 cM that is required for proper expression of dnc +. The dncck mutation, associated with a reciprocal translocation between 3L and the X, exhibits reduced form II activity and female sterility. This translocation breakpoint has been mapped to the left of the dnc + gene and is near the breakpoint of Df(1)N 64j15 which also reduces expression of dnc +. The effect of these independent chromosomal breaks on the dnc + gene suggests the existence of a site to the left of dnc + that is also required for proper expression of the gene.