In order to clarify the effect of nighttime temperatures on fruit development of Satsuma mandarin, we examined the fruit water and carbon balances using the 13C tracer method and the roles of phloem and xylem transports for fruit growth under moderate night temperatures (MN, set at 23°C) and low night temperatures (LN, set at 13°C). The average predawn xylem water potentials were -0.79 ± 0.04 MPa under MN and -0.77±0.03 MPa under LN. Fruit growth used 86% of pedicel sap flux toward the fruit, while transpiratory water losses from the fruit surface were 14% of pedicel sap flux under both MN and LN. The daytime integrated xylem sap flux was negative, but it was positive in the nighttime. The integrated phloem sap flux (ΣJPhlo) and the difference in ΣJPhlo, between MN and LN were only' 6-10% and 4% of the total sap accumulated in the fruit, respectively. Integrated fruit photosynthesis and integrated CO2efflux from the fruit surface were 7-8% and 22-23% of the total carbon supply toward the fruit, respectively. This indicates that carbon translocation from leaves to fruit via the phloem of the stem is the main source of carbon for the fruit.