In mammalians, Glucose 6-phosphate dehydrogenase (G6PDH) is clearly related with the daily cycle lipogenesis/lipolysis and therefore with the photoperiod and activity. In fish the temporally changes on the activity of this enzyme have not been described. In this work, annual and diurnal activity of G6PDH was investigated in the liver of Sparus aurata in different seasonal conditions of temperature and photoperiod. During experiment the fish were kept in cages under natural temperature and photoperiod while consuming a commercial diet. The months studied were chosen to establish if there is any influence on the G6PDH activity, because of the change in temperature and photoperiod (equal photoperiod and different temperature, March and October; different photoperiod and equal temperature, May and November; and different photoperiod and temperature, June and January). To study the enzyme activity along the day, the livers of six fish were removed every 3 hr during 24 hr. Annual activity was determined as the average activity during 1 day. Also the annual changes on fat content in muscle and digestive system were studied. The Vmax (maximum velocity) showed seasonal changes in period and phase of the rhythms and for the Km (Michaelis-Menten constant) a period of 24 hr was found in all sampled months, except March, the phase time moves depending on the month. In addition, the annual variations in muscular fat and G6PDH-specific activity seem to be more related to the reproductive stage than any other variable considered.