The anatomy of the basalar muscle and nerve of lamellicorn beetles is described, and a dissection is given for the preparation of the muscle for physiological experiments. The fibres of the muscle are singly innervated, but separate 'muscle units' are innervated by different nerve fibres, permitting graded action of the muscle as a whole. The neuromuscular junction shows facilitation and conforms to the 'slow' arthropod type; muscle potentials increase in amplitude on repetitive stimulation and show little or no overshoot of zero potential. These neuromuscular properties differ from those of previously described insect fibrillar muscles. Electrical activity in the basalar muscle during tethered flight of the whole insect confirms the presence of more than one muscle unit. It is shown that these properties are well adapted to the functioning of a muscle which both provides power for flight and is used in control adjustments.