Calmodulin is a highly conserved regulatory protein found in all eukaryotic organisms which mediates a variety of calcium ion-dependent signalling pathways. In the Drosophila retina, calmodulin was concentrated in the photoreceptor cell microvillar structure, the rhabdomere, and was found in lower amounts in the sub-rhabdomeral cytoplasm. This calmodulin localization was dependent on the NINAC (neither inactivation nor afterpotential C) unconventional myosins. Mutant flies lacking the rhabdomere-specific p174 NINAC protein did not concentrate calmodulin in the rhabdomere, whereas flies lacking the sub-rhabdomeral p132 isoform had no detectable cytoplasmic calmodulin. Furthermore, a defect in vision resulted when calmodulin was not concentrated in the rhabdomeres, suggesting a role for calmodulin in the regulation of fly phototransduction. A general function of unconventional myosins may be to control the subcellular distribution of calmodulin.