The dachshund gene encodes a transcription factor required for the proper development of Drosophila eyes, legs, and mushroom bodies. The mushroom bodies of dachshund mutants exhibit a marked reduction in the size of the vertical lobes and disorganization of the horizontal lobes. In mosaic animals, mutant axons fail to contribute significantly to the mushroom body alpha lobe. Here we show that this defect is due to the misrouting of alphabeta axons to the region normally occupied by alpha'beta' axons. This defect is pronounced for clones generated in larval stages but not clones generated after pupariation, indicating that dachshund function is particularly important around the time of puparium formation. In addition, mushroom body axons exhibit excessive branching in dachshund mutant clones. Thus, dachshund is required in mushroom body neurons for proper axon guidance and branching.