1. Overcrowding of crayfish in an aquarium often results in distal migration of the proximal retinal pigment while the animals remain in darkness. 2. Bubbling of carbon dioxide through the water surrounding the crayfish has the same effect. 3. When carbon dioxide is passed through the water at the rate of 3 liters per hour, distal migration of this pigment is complete at an oxygen content of 0.0 cc. per liter, a carbon dioxide content of 66.0 cc. per liter, and pH 5.3. 4. When carbon dioxide is passed through the water at the rate of 6 liters per hour, distal migration is complete at an oxygen content of 0.0 cc. perliter, a carbondioxide content of 277.1 cc. perliter, and pH 4.4. The speed of distal migration varies in general with the speed of administration of carbon dioxide. 5. When, instead of carbon dioxide, nitrogen is passed through the water at the rate of 2-l iters per hour, distal migration is complete at an oxygen content of 0.0 cc. per liter, a carbon dioxide content of 14.2 cc. per liter, and pH 7.3. 6. Acidification of the water with HCI, so long as the oxygen supply remains normal, produces no discernible effect on distal migration, except in the pH range 3.3-3.9, which is considerably below that involved in the preceding experiments. 7. The only factor shown to have a definite relation to distal migration in these experiments is oxygen deficiency. Distal migration is always complete when the oxygen content of the surrounding water reaches zero. 8. The various secondary factors promoting distal migration of the proximal retinal pigment—viz, low temperature, oxygen deficiency, anaesthesia, and death—all tend to retard metabolic activity. The evidence indicates that distal migration of this pigment, while influenced primarily by the presence of light, may secondarily be induced by agents which reduce the metabolic rate.