Photophores in Porichthys notatus consist of a cellular lens, an underlying area of photogenic tissue and a deep ensheathing reflector. Lens cells exhibit a dense compacted filamentous cytoplasm. Their processes interdigitate and desmosomes exist along their borders. The photogenic tissue, richly supplied by blood vessels, consists of two main cell types: (1) a presumed “photogenic” cell; and (2) a supportive cell. Photogenic cells are characterized by a highly vesiculated cytoplasm and peripheral microvilli. Frequently, they exhibit lamellar membranous whorls. Some whorls display cytoplasmic cores with vesicles. Many of these vesicles communicate via pores with an extracellular channel that envelops the cells. Supportive cells contain cytoplasmic filaments and extend processes around the photogenic cells. Except for isolated desmosomal contact points, a wide extracellular channel intervenes between supportive and photogenic cells. A prominent basal lamina separates supportive cells from the surrounding connective tissue. The strongly birefringent reflector is composed primarily of cells containing guanine crystals. The crystals lie stacked in groups, each membrane-bounded crystal being separated from its neighbor by an intervening layer of cytoplasm. Such an arrangement produces constructive interference and accounts for the high reflectivity of this multilayered structure. Possible relationships of the above structural features and the production of light are discussed.