A brief account is given of observations of the fine structure of the insect ear by light microscopy. The observations generally confirm those of previous workers. The electron microscope has revealed many new structures in the ganglion. Problems of the structure of the neuron and its sensory hair, the scolopale, and the relationships of the membranes of the neuron and the various accessory cells, all so little understood by light microscopy, can now be resolved. The locust has a pair of ear drums, situated one on each side of the first abdominal segment. The auditory ganglion is attached to the inner surface of the drum and it contains about seventy bipolar sensory neurons. A sensory neuron, wrapped in a series of three satellite cells, together with an attachment cell, constitutes a sensory unit (sensillum). The sensory units lie inside the capsule of the ganglion and they are attached to the hypodermis of special processes of the drum. The apex of the dendrite of each neuron bears a cilium that contains nine fibrils, orientated longitudinally and concentrically. A root (a rod-shaped structure) runs downwards from the base of the cilium within the dendrite, and splits repeatedly into thirty or more rootlets. Both root and rootlets have a cross-striation of pale and dark bands of different widths. The result is a periodicity which is polarized or asymmetrical. The upper part of the root terminates in nine concentric projections. It is these processes that actually touch the base of the cilium. They enclose a structure termed a root apparatus. The distal part of the dendrite and the cilium project into a scolopale, the body of the cilium being contained in a wide extracellular compartment within the scolopale. The upper region of the cilium has a dilatation containing fibrous structures that are not present in the other parts of the cilium. The tip of the cilium projects into a channel in the scolopale cap. The cap, unlike the other components of the scolopale, is an extracellular structure. By light microscopy the scolopale appears as a tubular structure with a narrow entrance and is sealed at the other end by the cap. Electron microscopy shows that the wall of the scolopale is composed of six or more concentric rods of fibrous material. At the entrance the rods lie close together around the dendrite. Near the cap the rods are also close together and become fused into a tube, into which the cap fits like a lid. In the central region the scolopale rods lie farther apart. The rods are intracellular structures of the scolopale cell (see below). In this central region only a thin layer of scolopale cell cytoplasm and membrane surrounds each rod so that the extracellular compartment around the cilium extends outwards between the columns of cytoplasm containing the scolopale rods. There are three satellite cells. (a) The Schwann cell, whose processes encapsulate the cell body and axon of the bipolar neuron. (b) The fibrous sheath-cell, so-called because it contains fine fibrous material; it is wrapped round the basal region of the dendrite. (c) The scolopale cell, which is wrapped round the upper region of the dendrite, round the extracellular compartment containing the cilium, and contains the scolopale rods in the inner region of its cytoplasm. Finally, the scolopale cell is linked to the hypodermis of the drum by the attachment cell. The auditory nerve carries the axons of the bipolar cells from the ganglion to the central nervous system. Each of these axons has a separate sheath composed of interlocking folds of a Schwann cell. A second group of afferent axons, probably originating from hair receptors of the drum, are also found in the auditory nerve. They do not have individual sheaths, but run in bundles within folds of the Schwann cell.