ABSTRACT Isolated photophores of Maurolicus muelleri, maintained in saline at 20 °C, consume oxygen at a mean rate of 1·25 ± 0·07 nmol O2 min-1(N = 50). In the presence of 5 × 10−6 mol l−1 KCN, seven preparations did not luminesce while two showed only dim luminescence. In all nine preparations, the resting oxygen consumption decreased by about 50%. In the presence of 5× 10−s mol l−1 KCN, all of the photophores produced a slow, low luminescence and their oxygen consumption decreased by about 75%. In the presence of 5× 10−4mol l−1 KCN, all of the photophores produced a slow, high luminescence and their oxygen consumption decreased rapidly by about 92%. It is suggested that the oxygen needs for light production by the isolated photophores of the mesopelagic fish, Maurolicus, differ from those of the epipelagic fish, Porichthys.