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Drosophila phototransduction is mediated by phospholipase C leading to activation of cation channels (TRP and TRPL) in the 30000 microvilli forming the light-absorbing rhabdomere. The channels mediate massive Ca 2+ influx in response to light, but whether Ca 2+ is released from internal stores remains controversial. We generated flies expressing GCaMP6f in their photoreceptors and measured Ca 2+ signals from dissociated cells, as well as in vivo by imaging rhabdomeres in intact flies. In response to brief flashes, GCaMP6f signals had latencies of 10-25ms, reached 50% F max with ∼1200 effectively absorbed photons and saturated (ΔF/F 0 ∼10-20) with 10000-30000 photons. In Ca 2+ free bath, smaller (ΔF/F 0 ∼4), long latency (∼200ms) light-induced Ca 2+ rises were still detectable. These were unaffected in InsP 3 receptor mutants, but virtually eliminated when Na + was also omitted from the bath, or in trpl;trp mutants lacking light-sensitive channels. Ca 2+ free rises were also eliminated in Na + /Ca 2+ exchanger mutants, but greatly accelerated in flies over-expressing the exchanger. These results show that Ca 2+ free rises are strictly dependent on Na + influx and activity of the exchanger, suggesting they reflect re-equilibration of Na + /Ca 2+ exchange across plasma or intracellular membranes following massive Na + influx. Any tiny Ca 2+ free rise remaining without exchanger activity was equivalent to <10nM (ΔF/F 0 ∼0.1), and unlikely to play any role in phototransduction., (Copyright © 2017 The Authors. Published by Elsevier Ltd.. All rights reserved.)