1. The Daphnia compound eye movements can be driven by a flashing light.
2. The action spectrum for the threshold light intensity required to evoke this response depends on the orientation of the stimulus light beam with respect to the animal.
3. If the light falls on the eye through the top of the animal's head the action spectrum peaks at the low wavelength end of the spectrum, while if it falls on the eye through the side of the head the peak is in the yellow-green.
4. Eye movements cannot be evoked by illuminating any part of the animal except the compound eye so neither of these action spectra is due to a light receptor other than the compound eye.
5. Some anomalous action spectra in the literature on the behaviour of free-swimming Daphnia are accounted for.