In Drosophila, the enzyme dopa decarboxylase (DDC) catalyzes the last step in the biosynthesis of serotonin (5HT) and dopamine. In the present study, the distribution of DDC and one of its biosynthetic products (5HT) was determined in CNSs that were genetic mosaics, composed of neurons that have DDC activity and neurons that lack DDC activity. Phenotypic mosaicism, that is, immunoreactive (IR) and non-IR neurons within the same nervous system, was observed for both DDC and 5HT immunoreactivity. DDC-IR neurons were always 5HT-IR, but some 5HT- IR neurons devoid of DDC immunoreactivity were also observed. 5HT-IR neurons devoid of DDC immunoreactivity were always in close apposition to other DDC-IR neurons. We suggest that in vivo uptake mechanisms are responsible for 5HT accumulation in neurons devoid of DDC immunoreactivity.