1. The electroretinogram of Lucilia sericata has been recorded whilst the fly was stimulated by an approaching object and by a slow change of light intensity.
2. The approach of a dark object against a light background was accompanied by the gradual development of a negative potential which decayed rapidly when movement ceased.
3. The size of the potential change was found to be directly proportional to (a) the number of ommatidia stimulated by the approaching object, (b) the rate of the decrease and (c) the amount of the decrease of intensity which it produced.
4. A similar response could be produced by stimulating the fly with a decrease of intensity, the negative potential developed being directly proportional to the number of ommatidia stimulated, to the rate and to the amount of the decrease.