In sperm of echiuroid, oyster and sea urchin, which had been incubated in sea water at 20°C for 15 hr, the respiratory rate, which was lower than in fresh sperm, was enhanced by light at 430, 530 and 570 nm more strongly than at the other examined wavelengths. In fresh sperm, respiration was not affected by light. This incubation resulted in marked decrease in the activity of NADH cytochrome c reductase and the amount of cytochrome b in these sperm, more evidently than appreciable decrease in the cytochrome c oxidase activity and the amount of cytochrome aa₃. NADH cytochrome c reductase was activated by light with peaks of photo-activation at 430, 530 and 570 nm in fresh and incubated sperm. A marked decrease in the cytochrome b amount during incubation probably makes the reactions, such as catalyzed by NADH cytochrome c reductase, rate-limiting in mitochondrial respiratory chain, to reduce the respiratory rate in incubated sperm. Photo-activation of these enzyme reactions, in which cytochrome b is involved, seems to enhance the respiratory rate, only when they become rate-limiting. Tunicate sperm, which NADH cytochrome c reductase was insensitive to light, did not exhibit photo-activation of respiration, even after a long time incubation.