In the walking legs of the shore crab, Carcinus maenas, is a series of chordotonal organs. Each organ consists of a strand of elastic connective tissue in which are embedded scolopidia. The anatomy and histology of the organs in the coxopodite-basipodite, meropodite-carpopodite, carpopodite-propodite and propodite-dactylopodite joints are described in detail, as seen by light and electron microscopy. The organs are hereafter referred to by the initial letters of the leg segments with which they are associated. The CB organ runs from a projection near the dorsal hinge of the coxa to the dorsal rim of the basipodite. MC1 runs from the side of the tendon of the `accessory flexor' muscle to two attachments on the preaxial wall of the meropodite. MC2 runs from the adductor tendon to the preaxial wall of the carpopodite. CP1 runs from the productor tendon to two ventral attachments on the carpopodite. CP2 runs from the reductor tendon to the floor of the propodite. PD runs from the adductor tendon to the postaxial wall of the dactylopodite. The scolopidia have a tube distal to the scolopale, into which are inserted the ends of the distal processes of bipolar sensory nerve cells. The tube is an extracellular organ apparently formed by the cell that contains the scolopale as an intracellular organ. Each scolopidium has associated with it two sensory cells, whose cell bodies lie in, on or near the connective tissue strands. In CB the sensory cells of a pair are similar to one another (isodynal scolopidia); in the other organs the two cells are dissimilar in their fine structure (heterodynal scolopidia). The difference, in the heterodynal scolopidia, consists in the presence or absence of a part of the distal process, the ciliary segment, which has nine double peripheral filaments regularly spaced, and in the precise form of the distal end of the axial filament. In all scolopidia, the two distal processes of the sensory cells are separated by intrusions of the sheath cells or of the scolopale cell, except for an area near the base of the scolopale where their cell membranes are in apposition; this area is referred to as the ephapse. At the level of the base of the scolopale the distal processes each contain an axial filament, which shows transverse striations, and there are attachment plaques between the distal processes and the scolopale cell. Distal to this level, each sensory cell contains a centrosome. Distal to the centrosome, the distal processes, which cross the scolopale space to end in the tube, can be divided into the following regions: a ciliary segment (where it occurs), a paraciliary segment characterized by nine double peripheral filaments less regularly arranged than in the ciliary segment, and a terminal segment characterized by numerous single microtubules. It is suggested that in each scolopidium one sensory cell responds to extension of the strand, and one to its shortening. This might account for the unidirectional responses observed in the organs. No structural basis for the observed differentiation of the sensory cells into `position' and `movement' receptors could be found.