ABSTRACT.AverageDaphniaeye movement rates decline upon exposure to bright white light from an initial 8 Hz to 4.5 Hz after 20 min. Eye movement rates were maximal for white light of intensity 10W/m2, and for blue light presented through the side of the head, though no difference could be measured between the responses to different light colours if presentation was through the top of the head.Slow moving light stimuli rotating around the animal evoked different stereotyped eye movement patterns for blue and for orange light. Ultrastructural changes in the rectangular cross‐section rhabdom were confined to the central region if the animal was exposed to strong orange light, and to the short edges if exposed to strong blue ligh