In many of the photophores found in deep-sea fishes and invertebrates, light filters containing pigments lie between the tissues that generate light and the sea. The loss of light within such filters has been measured throughout the visible spectrum for a variety of animals. These filters differ greatly in their spectral absorption characteristics and do not all contain the same pigments. All those from ventral photophores have a transmission band in the blue corresponding to the daylight that penetrates best into oceanic waters. For two fishes it is shown that the light generated inside their photophores is a relatively poor spectral match for the ambient submarine daylight while the light emitted into the sea, after passing through the filters, is a good match. For a third fish a similar improvement in `colour match' is brought about not by passing the light through a filter containing pigments but by reflecting the light into the sea by a blue mirror. All these observations support the hypothesis that the ventral photophores are used for camouflage. Malacosteus niger Ayres 1848 is an oceanic fish which emits red light from a large suborbital photophore. The red light generated inside the photophore is largely absorbed by a coloured filter over its external surface which transmits only a band of light of wavelengths around 700 nm. This is a waveband which is heavily absorbed by oceanic sea water. It is shown, however, that animals that can emit and are sensitive to such far-red light will have very great advantages in being able to see without being seen. The ranges over which such red light can be useful for vision are, however, relatively small. The nature of the pigments found in these various photophores is discussed. It is also calculated that the intensities of penetrating daylight are such that visual acuity could be fairly good down to considerable depths in the mesopelagic zone.