Developmental studies have shown thatelavJ1mutants are temperature-sensitive for defects in retinula cell and optic lobe structure. Anatomical and physiological examination of mutants, shifted between permissive and restrictive temperatures, shows that the early pupal period is phenocritical for producing defects in retinula cells. Although this period precedes visible development of the rhabdomere, brief pulses to 29°C during this time result in characteristic abnormalities in rhabdomere structure. These studies also show that the temperature-sensitive effects on mutant optic lobe development begin prior to the effects on the retina, suggesting that theelavproduct participates directly in optic lobe as well as in retinula cell development.