One-day old mutant flies maintained under a 12hr light/12hr dark cycle show little if any decrease in the size of the rhabdomeres. After rearing mutant flies for 7 days under such a cycle, the rhabdomeres almost completely degenerate. When the retina of mutant animals are exposed to 0.5 sec pulses of orange light and the photoresponse measured, A significant delay is seen in the deactivation of the light response. These flies also exhibit a defective, prolonged depolarisation afterpotential (PDA), that is less than 10% seen in controls.