The Drosophila mushroom bodies (MBs), paired brain structures composed of vertical and medial lobes, achieve their final organization at metamorphosis. The alpha lobe absent (ala) mutant randomly lacks either the vertical lobes or two of the median lobes. We characterize the ala axonal phenotype at the single-cell level, and show that the ala mutation affects Drosophila ethanolamine (Etn) kinase activity and induces Etn accumulation. Etn kinase is overexpressed in almost all cancer cells. We demonstrate that this enzymatic activity is required in MB neuroblasts to allow a rapid rate of cell division at metamorphosis, linking Etn kinase activity with mitotic progression. Tight control of the pace of neuroblast division is therefore crucial for completion of the developmental program in the adult brain.