Dark respiration (non-photorespiratory mitochondrial respiration), which occurs both in the light and in darkness, is vital for growth and survival of algae and plays a critical role in modulating the carbon balance of them. In the present study, we have investigated dark respiration in the light (RL) and in darkness (RD) in three marine macroalgal species, Hizikia fusiformis (phaeophyta), Gracilaria lemaneiformis (Rhodophyta) and Ulva lactuca (Chlorophyta), cultured at 20 ℃ using aeration with two CO2 conditions: current ambient (CO2 concentration about 380 μl/L) and elevated CO2 (approximately 720 μl/L) air. RL was estimated by using the Kok method, whereas RD was determined as the rate of O2 influx at zero light. The results showed that both RL and RD were unchanged for the elevated CO2-grown algae relative to ambient CO2 concentration for all the algal species tested. However, RL was significantly lower than RD across all the algal species and growth CO2 treatments, demonstrating that daytime respiration was partly depressed by the light. The percentage of inhibition of respiration by light was similar between ambient and elevated CO2-grown algae. The ratio of respiration to photosynthesis, which tended to decrease when estimated using RL instead of RD, was not altered for the elevated relative to ambient CO2 concentration. The results suggest that RL, rather than RD, is a more accurate estimate of nonphotorespiratory carbon loss in marine macroalgae during the daytime. It would not be anticipated that elevated atmospheric CO2 would exert a substantial influence on respiratory flux either in the light or in darkness in these particular marine macroalgal species.