The spectral sensitivity of the ocelli of Calliphora was determined by means of the electroretino-gram (ER G). Sensitivity is high in the spectral range from 340 to 450 nm ; with still increasing wavelengths it rapidly decreases. Analysis of the shape of the ERG resulting from stimuli of different wavelengths, as well as experiments with selective chromatic adaptation, indicate that there is mainly one spectral mechanism contributing to the response.