The vertical distributions of this marine copepod were quantitatively detd. under different conditions of light both in nature and in the laboratory. The distributions in nature suggest that downward migration may be brought about by increasing light intensity while upward migration may be brought about by decreasing light, and some instances were found suggesting that the more rapid the change of intensity, the greater the response induced. The laboratory exps. suggest that this copepod requires changes of intensity to evoke vertical migration and the more frequently the changes occur the more the animals are stimulated to move either toward or away from the light as the case may be; furthermore, the range through which the changes of intensity occur may determine the response.