Cuticular stress detector afferents (CSD1 and CSD2) in the walking legs of crayfish have been studied in an in vitro preparation allowing intracellular recordings of single neurones during mechanical stimulation of the organs. Lucifer yellow staining of single fibres showed them to be dye-coupled through junctions presumably located near their point of entry into the ganglion. In intracellular recordings from identified sensory fibres with 30-80-mV impulses, coupled afferent spikes were seen as depolarizing potentials of short duration but smaller amplitude. Coupling sometimes involved fibres with quite different response characteristics to mechanical stimuli. Coupled spikes summated during strong mechanical stimuli. In silent CSD fibres, antidromic spikes could occur which were triggered by the depolarization due to coupling. Antidromic spikes were more frequent during strong stimuli and were in some cases shown to be triggered in a cooperative way when two afferent spikes synchronously reached the ganglion. Our results suggest that central coupling of primary afferents may be a widespread phenomenon in Crustacea and that it provides a threshold mechanism for selective amplification of strong stimuli.