<p>Guanine-type choroidal tapetum comprises a palisade of cells containing stacks of guanine crystals within the choroid, reflecting light from behind the photoreceptors and resulting in increased sensitivity. Although many cartilaginous fishes possess a guanine-type choroidal tapetum, such structures have at no time been investigated in Hexanchiformes. We investigated a quantitative chemical analysis of choroidal tapeta in two deep-sea hexanchiform sharks, the frilled shark <i>Chlamydoselachus anguineus</i> and sharpnose sevengill shark <i>Heptranchias perlo</i>. Both sharks exhibited eyeshine and contained silver choroidal tapeta in the whole area. The locus tapetalis, with guanine values exceeding 1.27 mg/cm² in <i>C. anguineus</i> and 0.49 mg/cm² in <i>H. perlo</i>, was developed in the temporal fundus in both sharks, the results suggesting that such facilitates naso-lateral detection with higher photosensitivity for predation in the deep-sea environment. The possible ecological advantages of the tapetum in hexanchiform sharks and their evolutionary relationship among other cartilaginous fishes are discussed.</p>