To investigate the role of light in controlling copepod grazlng behavlor, feeding responses of adult female Acartia tonsa Dana were determined under light conditions similar to those in natural, subsurface waters. Measured feeding responses were then related to feeding behavior under similar light conditions in nature. Copepods were adapted to a range of light intensities before being offered food (Thalassiosira weissflogii, 0.5 mm3 1 -l ) . Thirty min later gut contents were measured by gut fluorescence of chlorophyll a and pheopigment a. The grazing index was inversely related to the light intensity at which the copepods were adapted. The most dramatic change in grazing occurred at light levels close to those found in the copepod's natural habitat during twilight. An endogenous nocturnal feeding rhythm was found in a separate experiment. A 96 h field study in the Newport River estuary, North Carolina, USA, revealed a nocturnal feeding pattern in A. tonsa and a negative correlation between feeding and light intensity during daylight hours. No significant correlations were found between copepod feeding and in situ concentrations of chlorophyll a, pheopigment a and combined pigments. These results challenge Gauld's (1953) hypothesis that nocturnal grazing results from nocturnal vertical migration into a food-rich layer; they offer a reasonable explanation of how light may time grazing. Results indicate that migration and feeding may be independent behaviors separately controlled by light cues.