Four putative adenylyl cyclase genes from Drosophila melanogaster were identified by virtue of their extensive sequence homology with mammalian cyclases. One corresponds to the learning and memory gene rutabaga and is most similar to the mammalian brain Ca2+ calmodulin (CaM)-responsive cyclase. In a mammalian expression system, rutabaga cyclase activity was stimulated approximately 5-fold by the presence of Ca2+ CaM. A point mutation, identified at this locus in rut1 mutant flies, resulted in loss of detectable adenylyl cyclase activity. New P element insertion-induced rutabaga mutations mapped to within 200 nucleotides of the 5′ end of the rutabaga cDNA. These data confirm the identity of the rutabaga locus as the structural gene for the Ca2+ CaM-responsive adenylyl cyclase and show that the inactivation of this cyclase leads to a learning and memory defect.