Phototransduction in Drosophila occurs through inositol lipid signaling that results in Ca2+ mobilization. In this system, we investigate the hitherto unknown physiological roles of calmodulin (CaM) in light adaptation and in regulation of the inward current that is brought about by depletion of cellular Ca2+ stores. To see the effects of a decreased Ca-CaM content in photoreceptor cells, we used several methods. Transgenic Drosophila P[ninaCDeltaB] flies, which have CaM-deficient photoreceptors, were studied. The peptide inhibitor M5, which binds to Ca-CaM and prevents its action, was applied. A Ca2+-free medium, which prevents Ca2+ influx and thereby diminishes the generation of Ca-CaM, was used. The decrease in the Ca-CaM level caused the following effects. (i) Fluorescence of Ca2+ indicator revealed an enhanced light-induced Ca2+ release from internal stores. (ii) Measurements of the light-induced current in P[ninaCDeltaB] cells showed a reduced light adaptation. (iii) Internal dialysis of M5 initially enhanced excitation and subsequently disrupted the light-induced current. (iv) An inward dark current appeared after depletion of the Ca2+ stores with ryanodine and caffeine. Importantly, application of Ca-CaM into the photoreceptor cells prevented all of the above effects. We propose that negative feedback of Ca-CaM on Ca2+ release from ryanodine-sensitive stores mediates light adaptation, is essential for light excitation, and keeps the store-operated inward current under a tight control.