The fine structure of a joint receptor (R10) in a spider leg (Zygiella x-notata) was examined with light and electron microscopy. The R10 receptor consists of a compact ganglion which is situated near the dorsal joint membrane of the femur/patella joint. Each of the ten sensory cells comprising the ganglion sends one branching dendrite into the hypodermis underlying the joint membrane. All dendritic branches together form a sheet-like meshwork 50 microns wide and 1 microns thick, which is traversed obliquely by hypodermis cells. When the joint is stretched shearing forces are apparently transmitted to the receptive dendritic branches via microtubular bundles inside the hypodermis cells. The soma and dendrites of the sensory cells receive numerous synaptic input from presumably efferent fibres. The fine structure of these synapses is described and compared with other peripheral and central spider synapses. All R10 synapses contain small synaptic vesicles (32 nm diameter), whereas motor endplates possess large vesicles (38 nm). Central synapses have two significantly different vesicle populations which are either of the small or large variety. Since synapses with small vesicles are supposedly inhibitory, receptor cells in spiders might be under efferent control. Such a system is unknown in insects or crustaceans, but may be typical for arachnids.