The exoskeletal surface of the body of the terrestrial isopod, Porcellio scaber has been examined with the scanning electron microscope as part of a study of the structural changes associated with the transition from sea to land in the Isopoda. No sexual differences are apparent. The dorsal cuticular surface is characterized by large tubercles, minute plaques and pits, separated by amorphous areas. Apart from the eyes the only structures of apparent sensory function are relatively large setae which occur near the antero-lateral and posterolateral edges of each epimeron, and sensilla squamiformia (tricorns). Tricorns are extremely numerous on the dorsal surface and appear to be formed from the fusion of a plaque and a sensillum which arises from a pit. Much of the ventral body surface is covered with closely-set plaques, tricorns being restricted to the epimera. Along the sides of the mid-ventral sternal groove occur numerous small tubercles each surmounted by a single seta. Similar, though smaller structures are found in a single row along the outer edge of the epimera. The water capillary system of this species is formed from spines which arise from plaques around the pereopodal sockets and along the junctions between the epimera and sternites. Most of the sensilla described appear to be mechanosensory and are probably associated with the thigmokinetic and proprioceptive responses of the woodlouse.