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Abstract Serotonergic neurons produce extensively branched axons that fill most of the central nervous system, where they modulate a wide variety of behaviors. Many behavioral disorders have been correlated with defective serotonergic axon morphologies. Proper behavioral output therefore depends on the precise outgrowth and targeting of serotonergic axons during development. To direct outgrowth, serotonergic neurons utilize serotonin as a signaling molecule prior to it assuming its neurotransmitter role. This process, termed serotonin autoregulation, regulates axon outgrowth, branching, and varicosity development of serotonergic neurons. However, the receptor that mediates serotonin autoregulation is unknown. Here we asked if serotonin receptor 5‐HT1A plays a role in serotonergic axon outgrowth and branching. Using culturedDrosophilaserotonergic neurons, we found that exogenous serotonin reduced axon length and branching only in those expressing 5‐HT1A. Pharmacological activation of 5‐HT1A led to reduced axon length and branching, whereas the disruption of 5‐HT1A rescued outgrowth in the presence of exogenous serotonin. Altogether this suggests that 5‐HT1A is a serotonin autoreceptor in a subpopulation of serotonergic neurons and initiates signaling pathways that regulate axon outgrowth and branching duringDrosophiladevelopment.