The feeding of turbot during the on-growing phase is done with highly proteinaceous feeds, raw fish and fish meal being the main ingredients. The protein of these feeds is used inefficiently because the amino acids of the protein are metabolised to produce energy instead of contributing to protein growth (Cacho et al. , 1990). Alternative sources of energy, such as carbohydrate, are known to spare protein (Hepher, 1986). The aim of this experiment was to test the idea that fish given free access to feeds with different protein: carbohydrate ratios will (a) eat enough feed to meet their protein requirement and (b) maximise their efficiency of use of protein for growth when a sufficient supply of an alternative source of energy (carbohydrate) is available.