Using a double-staining technique with the neuron-specific monoclonal antibody 22C10 and the anti-Ubxmonoclonal antibody FP 3-38 we describe the development of landmark cells in the nervous system ofDrosophila. The staining with MAb 22C10 provides an internal system of reference that allows a precise localization of the most prominentUbxactive cells. The expression ofUbxseems to initiate at the segmental border in the hypoderm and the homologous region in the neuromeres. Also, the extent ofUbxexpression follows quantitative and qualitative changes during embryonic development.