We examined the embryonic development of an identified group of thoracic spiking local interneurones in the locust. These interneurones are primary integrators of mechanosensory information from the legs and make inhibitory output connections with motor neurones, nonspiking local interneurones, and intersegmental interneurones. Using intracellular dye injection, we traced the origins of these interneurones and described their morphological development. All of the interneurones in this population are produced by neuroblast NB4-1. The interneurones are produced during the latter stages of the neuroblast lineage and could not be identified before 55% development. Their growth could be divided into three distinct phases: first, a period of initial outgrowth (55-70%) during which the basic skeleton of major neurites is formed; second, a shorter period of rapid growth (70-80%) during which the basic skeleton is elaborated by the addition of many side branches; and third, a period of maturation (80-95%) during which the branches formed during earlier growth appear to be pruned. The possible implications of these results for neural development and synaptogenesis are discussed.