The potential for any morphological part of a photoreceptor cell to function as an optical waveguide is determined 1) by the dimensionless parameter V as shown by Snyder (IEEE Trans., Microwave Theory Tech. 17: 1133, 1969) and by Snyder, Pask and Mitchell (J. Opt. Soc. Am. 63: 59, 1973), and 2) by the nature of the optical excitation for that part of the cell. The effects of these two factors are considered for the myoids and outer segments of vertebrate photoreceptor cells and for the crystalline tracts and rhabdomers of invertebrate photoreceptor cells. Examples of both guiding and nonguiding myoids, outer segments, rhabdomers and tracts are cited and some consequences for vision are analyzed.