We studied the mechanism of pigment granule migration in fly photoreceptors. We found that these granules migrate transiently with red light stimulation. This suggests that excitation of metarhodopsin triggers granule migration. We also found that cyclic GMP is one of the activators of pigment granule migration. These findings suggest that pigment granule migration is triggered separately from phototransduction, which is initiated by the excitation of rhodopsin.