The antennal lobes (ALs), the primary olfactory centers, of the moth Manduca sexta are sexually dimorphic. Only ALs of males possess the macroglomerular complex (MGC), the site of primary processing of information about the female's sex pheromone. To understand the development of identified, odor-specific olfactory glomeruli, we investigated the cellular events involved in the morphogenesis of the MGC by means of various fluorescence staining techniques and laser-scanning confocal microscopy. The MGC lies near the entrance of the antennal nerve into the AL of the adult male and comprises three glomeruli, the globular cumulus and two toroidal structures. The MGC forms during early stages of metamorphic adult development through a stereotyped sequence of coordinated changes in MGC-specific receptor axons, glial cells, and early-ingrowing projection neurons of the medial group of AL neurons. The MGC divisions are the earliest glomeruli to form in the male AL, and their basic organization is established within about 3 days after ingrowth of the first sensory axons. Despite their special anatomical features, the MGC glomeruli develop in a manner similar to that of the ordinary glomeruli. Comparison of the ALs of males and females reveals that two relatively large and early-developing glomeruli that are situated dorsolaterally in the female AL appear to be female-specific. Development of the sexually dimorphic glomeruli diverges immediately after the ingrowth of the first olfactory receptor axons, resulting in the formation of these large glomeruli in females and the MGC in males.