A population of ascending interneurones with cell bodies in the metathoracic ganglion of the locust is described. Interneurones are characterised by their morphology (revealed by intracellular cobalt injection) and by their physiological responses to afferent stimulation. All interneurones have their somata in the ventral cortex of the ganglion, in an area just posterior to the medial tracheae. On the basis of gross morphology the interneurones can be divided into three groups: (1) those that have a main area of fine neurites ipsilateral to the soma and an ipsilateral ascending axon; (2) interneurones that also have their main neurites ipsilateral to the soma but have a contralateral ascending axon, and (3) interneurones that have their main stimulation of sensory receptors on the hind leg ipsilateral to the main neurites. Interneurones receiving excitatory inputs from tactile hairs on the hind leg have branches in the most ventral neuropil, whereas interneurones receiving input from leg proprioceptors have branches in the more intermediate and dorsolateral neuropil. The branching pattern of an interneurone and the size and position of the receptive field on the leg are correlated. Interneurones with restricted branching patterns have restricted receptive fields. The position of the ventral branching reflects the position of the receptive field on the leg. An interneurone with a receptive field restricted to the femur has ventral branches in an anterior position in the ganglion; an interneurone with a receptive field restricted to the tarsus has ventral branches in a more posterior position.