Expression of the human Parkinson-disease-associated protein α-synuclein in all Drosophila neurons induces progressive locomotor deficits. Here, we identify a group of 15 dopaminergic neurons per hemisphere in the anterior medial region of the brain whose disruption correlates with climbing impairments in this model. These neurons selectively innervate the horizontal β and β' lobes of the mushroom bodies, and their connections to the Kenyon cells are markedly reduced when they express α-synuclein. Using selective mushroom body drivers, we show that blocking or overstimulating neuronal activity in the β' lobe, but not the β or γ lobes, significantly inhibits negative geotaxis behavior. This suggests that modulation of the mushroom body β' lobes by this dopaminergic pathway is specifically required for an efficient control of startle-induced locomotion in flies.