To scrutinize the common belief that the number of neurons in the CNS of adult decapod crustaceans stays constant, in spite of their dramatic postlarval increase in size, I counted olfactory projection neurons (OPNs) in the brains of differently-sized postlarval shore crabs, Carcinus maenas, and performed in vivo labeling of proliferating cells with 5-bromo-2'-deoxyuridine (BrdU) on brains of adults. The number of OPNs increases continuously throughout the postlarval life of shore crabs and approximately doubles from the very young to the oldest animals. Brain sections from adult crabs labeled with BrdU revealed ongoing proliferation of cells in the lateral soma cluster, which consists of OPN cell bodies, and in the cluster of somata of hemiellipsoid body local interneurons, which are the targets of the OPNs. Post-injection survival times from 5.5 to 120 h revealed a small but relatively constant number of labeled nuclei with neuronal morphology in both soma clusters of all specimens (31.3 +/- 9.5 S.D. nuclei per lateral cluster, n = 29; 20.1 +/- 4.5 S.D. nuclei per hemiellipsoid body cluster, n = 10). The labeled nuclei were located in a distinct proliferative zone in each cluster. There were significantly more labeled nuclei in both soma clusters after a prolonged post-injection survival time of 1 month (71.3 +/- 7.8 S.D. nuclei per lateral cluster, n = 4; 38.2 +/- 7.1 nuclei per hemiellipsoid body cluster, n = 6). In both soma clusters the labeled nuclei formed a compact group that was dislocated from the proliferation zone towards the outer edge of the cluster. In the proliferation zone of the lateral cluster histological stainings revealed cell bodies of typical neuronal shape that are slightly smaller and more intensely stained than the surrounding OPN somata. Some of these cell bodies were captured in various stages of mitosis. Collectively, these data indicate that continuous neurogenesis occurs in the central olfactory pathway of the brain of shore crabs throughout their entire adult life. This unexpected structural plasticity may enable long-lived decapod crustaceans to adapt to ever-changing olfactory environments.