ABSTRACT A muscle attached to the medial edge of the compound eye is described for the blowfly Calliphora-vomitoria. Electrophysiological activity in the form of continuous tonically firing potentials can be recorded extracellularly from the muscle. These potentials are generated by the muscle and have the same origin as the ‘clock-spikes’ recorded previously from the optic lobe of Calliphora erythrocephala. The interspike interval of the eye muscle potentials varies inversely with the ambient temperature. Light-adaptation results in a decrease and dark-adaptation an increase in the resting interspike interval of the eye-muscle potentials. Light-adaptation is correlated with increase and dark-adaptation with decrease in the depth of the compound eye as measured at the insertion of the muscle. The pseudopupil produced by illumination of the compound eye from the inside displays spontaneous movements which can be correlated with the anatomicall arrangement and spontaneous activity of the eye muscle. The probable function of spontaneous and transient changes in eye-muscle activity is to promote scanning of the visual images produced by the dioptrics of the compound eye.