The avian trigeminal ganglion, which is embryonically derived from the neural crest and epidermal placodes, consists of two topographically segregated classes of immature neurons, large and small, during the second week of incubation, and two neuronal cell types, dark and light, interspersed throughout the mature ganglion. In order to establish the times of terminal mitosis of trigeminal sensory neurons, embryos were treated with [3H]thymidine during the first week of incubation and their ganglia fixed on embryonic day 11. The embryonically large, distal, placodal-derived neurons are generated between days 2 and 5, while the small, proximal, neural crest-derived neurons are formed mostly between days 4 and 7. By comparing the locations of labeled cells in ganglia treated with isotope but fixed on day 18 of incubation with their 11-day counterparts, we have proved that there are no morphogenetic rearrangements of neurons during the final week of incubation. Thus, no unique relationship exists between the two neuron types in the mature ganglion and the two cell classes in the immature trigeminal. Therefore, both the light and the dark neurons in the mature trigeminal ganglion arise from neural crest as well as placodal primordia.