We suggest a way to estimate phytoplankton respiration separately from heterotrophic respiration. The analysis is based on three findings: (1) that carbon loss in the plankton overnight is mostly phytoplankton respiration; (2) that uptake of 14C estimates net primary production; and (3) that phytoplankton respiration occurs during the day at the same rate as at night. Based on these three findings, we propose that phytoplankton respiration can be estimated as twice the overnight loss of organic carbon. Heterotrophic respiration is then the difference between phytoplankton and total community respiration.