It has been hypothesised that, at non-limiting water oxygen conditions, voluntary feed intake (FI) in fish is limited by the maximal physiological capacity of oxygen use (i.e. an 'oxystatic control of FI in fish'). This implies that fish will adjust FI when fed diets differing in oxygen demand, resulting in identical oxygen consumption. Therefore, FI, digestible energy (DE) intake, energy balance and oxygen consumption were monitored at non-limiting water oxygen conditions in Nile tilapia fed diets with contrasting macronutrient composition. Diets were formulated in a 2 £ 2 factorial design in order to create contrasts in oxygen demand: two ratios of digestible protein (DP):DE ('high' v. 'low'); and a contrast in the type of non-protein energy source ('starch' v. 'fat'). Triplicate groups of tilapia were fed each diet twice daily to satiation for 48 d. FI (g DM/kg 0·8 per d) was significantly lower (9·5 %) in tilapia fed the starch diets relative to the fat diets. The DP:DE ratio affected DE intakes (P,0·05), being 11 % lower with 'high' than with 'low' DP:DE ratio diets, which was in line with the 11·9 % higher oxygen demand of these diets. Indeed, DE intakes of fish showed an inverse linear relationship with dietary oxygen demand (DOD; R 2 0·81, P,0·001). As hypothesised ('oxystatic' theory), oxygen consumption of fish was identical among three out of the four diets. Altogether, these results demonstrate the involvement of metabolic oxygen use and DOD in the control of FI in tilapia.Key words: Feed intake: Oxygen consumption: Non-protein energy sources: Energy balance: FishVoluntary feed intake (FI) in fish, as in other animals, is controlled by a complex combination of nutritional, physiological and environmental factors (1) . It has frequently been suggested that FI is controlled to maintain a relatively constant digestible energy (DE) intake, i.e. to meet the DE requirements (2 -6) .However, a few studies in fish have suggested the involvement of other nutritional factors in the control of FI (7,8) . Studies that verify the importance of other well-known regulatory mechanisms of FI in mammals, such as glucostatic (9) or lipostatic control (10) , either lead to an ambiguous conclusion or show a lesser impact in fish (7,11) than in terrestrial animals. The effect of non-protein energy (NPE) source (fat and starch) on FI in fish is unclear (12) and has not been systematically assessed at least at a similar digestible protein (DP):DE ratio and DE content of diets. Among the abiotic factors, dissolved oxygen (DO), pH and NH 3 are recognised to affect FI in fish (13) . The effect of the availability of oxygen on FI has been relatively well documented. Several studies have demonstrated that FI in fish decreases linearly with declining water DO content (14 -17) .The minimum DO level at which metabolic oxygen demand in fish limits FI is termed as incipient DO (iDO). Thus, * Corresponding author: J. W. Schrama, fax þ31 317 483937, email johan.schrama@wur.nlAbbreviations: ADC, apparent digestibility coefficie...