Cyclic AMP signaling in Drosophila mushroom body neurons, anchored by the adenylyl cyclase encoded by the rutabaga gene, is indispensable for olfactory memory formation. From a screen for new memory mutants, we identified alleles of the gilgamesh (gish) gene, which encodes a casein kinase Iγ homolog that is preferentially expressed in the mushroom body neurons. The gish-encoded kinase participates in the physiology of these neurons underlying memory formation since the mutant memory deficit was rescued with expression of a gish cDNA in these neurons only during adulthood. A cellular memory trace, detected as increased calcium influx into the α'/β' neuron processes in response to the odor used for conditioning, was disrupted in gish mutants. Epistasis experiments indicated a lack of genetic interactions between gish and rutabaga. Therefore, gish participates in a rutabaga-independent pathway for memory formation and accounts for some of the residual learning that occurs in rutabaga mutants.