Mutant flies show a twofold reduction in the number of pigment granules in secondary and tertiary pigment cells compared to wild type, although the pigment granules appear to be slightly larger than normal. There are fewer but larger pigment granules in the fenestrated layer. The width of the fenestrated layer between the basement membrane and the cellular cortex of the lamina monopolar cells is reduced. rb^D1/rb^S1, rb¹⁰/rb^S1 and Df(1)rb1/rb¹⁰ flies have a normal landing and optomotor response and respond normally to stationary visual stimulation. They show abnormal behaviour in Buridan's paradigm, a test for alternating pattern fixation.