The iridescent stripe of the freshwater teleost, the neon tetra, changes from green in the daytime to violet-blue at night. 2. Spectral reflectance measurements were used to follow these colour changes. 3. Light causes a shift in reflectance to longer wavelengths in living fish and in isolated tissue from the lateral stripe. The change is reversed in darkness. 4. The spectral reflectance shifts to longer wavelengths when the fish is disturbed in darkness. No such colour changes were seen in fishes kept alive in 10(-4) M-reserpine. 5. Hypotonic Ringer solution causes a reflectance shift to longer wavelengths and hypertonic solution causes a shift to shorter wavelengths. 6. The iridescent reflexions from the lateral stripe which is continued across the iris originate from iridophores in the dermis. These iridophores contain regular stacks of broad, double-sided hexagonal plates that are about 10 nm thick. Each plate is contained within a pouch in the cytoplasm and is separated from its neighbour by approximately one quarter the wavelength of light. 7. A distinction is drawn between the physiologically active iridophores in the lateral stripe and iris that have broad hexagonal crystal plates which are very thin and the physiologically inactive iridophores that are also found in the iris, but in addition are found on the flanks below the lateral stripe, and on the head. These iridophores contain hexagonal crystals that are usually narrower than the active type, but are about 60-100 nm thick.
