Invertebrate photoreceptors use the ubiquitous inositol-lipid signaling pathway for phototransduction. This pathway depends on Ca2+ release from internal stores and on Ca2+ entry via light-activated channels to replenish the loss of Ca2+ in those stores. The Drosophila transient receptor potential (TRP) protein is essential for the high Ca2+ permeability and other biophysical properties of these light-activated channels, which affect both excitation and adaptation in photoreceptor cells. Physiological and heterologous expression studies indicate that TRP is a putative subunit of a surface membrane channel that can be activated by depletion of internal Ca2+ stores. Furthermore, trp is an archetypal member of a multigene family whose products share a structure that is highly conserved throughout evolution, from worms to humans.