The present study is a morphological, biochemical and spectrophotometric characterization of the eye lens pigmentation in 45 specimens (11-88 mm in standard length) of the deep-sea hatchetfish, Argyropelecus affinis (Stomiiformes: Sternoptychidae). For comparison, we also examined available lenses of other members of the family Sternoptychidae, including three other species of the genus Argyropelecus, and two species of the genus Sternoptyx. Lens pigmentation was observed in all specimens of Argyropelecus spp. larger than about 36 mm in standard length, but was absent in all Argyropelecus spp. individuals less than 36 mm. However, lens pigmentation was not observed in Sternoptyx specimens of any size. Detailed studies of A. affinis indicated that at 36 mm the nascent lens fiber cells, which are continually laid down over preexisting, unpigmented cells, begin incorporating pigment, and the pigment concentration increases steadily as pigmented cells are added during lens growth. Spectrophotometric and biochemical data suggested that the pigment is a carotenoprotein complex, the carotenoid-like chromophore being strongly associated with a specific soluble lens protein, alpha crystallin. While the lens coloration in these fishes is age-related, analyses of the retinal visual pigment revealed no concomitant age-related change in the peak wavelength of retinal sensitivity in these fishes. Our data on the spectral absorbance of the lens and visual pigment of these fishes suggest that the lens pigmentation acts as a short-wave filter to improve acuity of the visual system.