To clarify why fruit bagging reduces sugar content at harvest, we investigated its effect on carbon dioxide assimilation by Satsuma mandarin (Citrus unshiu) fruit through photosynthesis and phosphoenolpyruvate carboxylase (PEPC; enzyme code 22.214.171.124). Seasonal changes in gross photosynthesis ranged from 70 to 400 μmol·d−2·h−1 O2 with a peak at 99 days after full bloom (DAFB) when the assimilation rate of fruit was comparable to that of leaves. However, a peak showing net photosynthesis appeared at 112 DAFB because of high fruit respiration. When fruit were bagged at 85 DAFB, the net photosynthetic peak disappeared, perhaps as a result of the decline in chlorophyll content in the rind. Sugar and organic acid content in the bagged fruit were 0.3% and 0.16% less, respectively, than controls at the mature stage (204 DAFB). PEPC activity in the rind was much higher than in leaves on a protein basis; it increased between 92 and 112 DAFB and showed a peak of 72 units. The PEPC activity peak was also 90% of control after fruit bagging. Thus, just before their color development, mandarin fruit assimilate CO2 actively through photosynthesis and PEPC. However, these activities are inhibited by bagging, likely resulting in lower sugar content at harvest. The concomitant activation of PEPC and photosynthesis between 99 and 126 DAFB indicates that CO2 fixed by PEPC might be used for photosynthesis in mandarin fruit, because photosynthesis in several fruit such as apple (Malus pumila) and pea (Pisum sativum) is considered to have an intermediate status among C3, non-autotrophic tissue, and C4/CAM photosynthesis.