Melatonin was radioimmunoassayed in extracts from brains of Musca autumnalis De Geer, the face fly, obtained 4h apart during 24h in 3 series pools of over 2 dozen brains at each of 6 or 7 circadian times. A circadian rhythm in the data expressed as percent of mean was demonstrated by the population-mean cosinor method. Face flies thus offer themselves for studies of melatonin-mediated chronomodulation by feedsideward mechanisms.