Rock oysters from a mass selection trial were compared with wild-caught (control) oysters of the same age to determine the physiological basis for faster growth rates amongst the selected individuals, and to describe the associated flexibility in phenotypic traits of feeding, metabolism and growth. In confirmation of earlier studies, fast growth was associated with faster rates of feeding, reduced metabolic rates and lower metabolic costs of growth. Selected individuals deposited more protein, at a lower metabolic cost, than the controls. Control oysters, however, deposited more lipid than the selected oysters, though the unit costs of lipid deposition did not differ between categories. The results indicated a wide plasticity of physiological rates and efficiencies and demonstrated how, by selection, interactions between physiological traits can serve to enhance growth. If differences in lipid deposition observed here were indicative of different rates of gametogenesis, then the results also suggest that selection alters the balance between growth and reproduction. Whether these differences can be termed compensatory with respect to the life history of the species remains to be determined, but the results indicate some of the ways in which physiological flexibility may be expressed to effect different patterns of energy allocation.