Daily variations in oxygen consumption by young Mediterranean yellowtails (Seriola dumerili) were measured under several feeding frequencies: three meals a day from Monday to Friday (STD), two meals a day on Saturdays (SAT) and Sundays (SUN) and fasted during 24 (FS1) and 48 h (FS2) prior to monthly fish body weight samplings. The average weight was 308 g and the fish were fed extruded pellets in open 2 m3 tanks. Initial biomass was around 4 kg per tank. Heated seawater was kept at 19±1 °C, its salinity was kept unspoiled at around 38 g l−1 and percentage of oxygen saturation of inlet water was close to 100%. The photoperiod was adjusted to 12-h light:12-h dark. A control tank without fish made it possible to minimise from the calculations the effect of oxygen diffusion between air and water. The average oxygen consumption (mg O2 kg−1 h−1 ±S.D.) in each experimental group was STD=219±78, SAT=204±66, SUN=213±63, FS1=152±36 and FS2=137±44. An increase in the oxygen consumption after the first meal was observed in all cases, although this effect seemed to be clearly influenced by the moment in which the last meal was supplied. After the first day of fasting, a significant decrease of oxygen consumption reaching basic levels was observed and this level was maintained during the second fasting day. No effect attributable to anticipation of food supply was noted. The rhythm acrophases detected in each group were significantly different except for the case of FS1 and FS2, where an overlap of confident areas was shown. In the case of FS1 and FS2, the Cosinor''s analysis indicated the existence of a significant circadian rhythm, but the percentage of this variance was smaller than in the other cases. [Copyright &y& Elsevier]