Human genetic studies support that loss-of-function mutations in theSH3 domain andankyrin repeat containing family proteins (SHANK1-3), the large synaptic scaffolding proteins enriched at the postsynaptic density of excitatory synapses, are causative for autism spectrum disorder and other neuropsychiatric disorders in humans. To better understand thein vivofunctions of Shank and facilitate dissection of neuropathology associated withSHANKmutations in human, we generated multiple mutations in theShankgene, the only member of the SHANK family inDrosophila melanogaster. Both male and femaleShanknull mutants were fully viable and fertile with no apparent morphological or developmental defects. Expression analysis revealed apparent enrichment of Shank in the neuropils of the CNS. Specifically, Shank coexpressed with another PSD scaffold protein, Homer, in the calyx of mushroom bodies in the brain. Consistent with high expression in mushroom body calyces,Shankmutants show an abnormal calyx structure and reduced olfactory acuity. These morphological and functional phenotypes were fully rescued by pan-neuronal reexpression of Shank, and only partially rescued by presynaptic but no rescue by postsynaptic reexpression of Shank. Our findings thus establish a previously unappreciated presynaptic function of Shank.SIGNIFICANCE STATEMENTMutations inSHANKfamily genes are causative for idiopathic autism spectrum disorder. To understand the neural function of Shank, a large scaffolding protein enriched at the postsynaptic densities, we examined the role ofDrosophilaShank in synapse development at the peripheral neuromuscular junctions and the central mushroom body calyx. Our results demonstrate that, in addition to its conventional postsynaptic function, Shank also acts presynaptically in synapse development in the brain. This study offers novel insights into the synaptic role of Shank.