A single cohort of small individuals (31 mm mean shell length, 112 mg mean dry flesh weight) of the marine bivalve mollusc Mytilus galloprovincialis Lmk. was held sequentially for 2 wk at each of four food levels equivalent to ingested rations of less than 0.1%, 2.6%, 3.1%, and 7.4% of dry body weight per day. Growth rate reached a maximum at the highest ration level and was strongly correlated, amongst individuals, with mean heterozygosity measured across nine enzyme loci. Rates of energy expenditure were analysed separately as maintenance metabolic rate and the energy costs of growth (J mg-1 dry tissue). The maintenance metabolic rate correlated with traits of protein metabolism (protein synthesis, deposition, and breakdown), and the separate energy costs of both maintenance and growth correlated with the efficiency of protein deposition (protein growth as a proportion of synthesis). The energy costs of growth also varied in negative relation to mean individual heterozygosity. In a multiple regression analysis, the energy allocation to the costs of growth, body size, mean heterozygosity, and the efficiency of protein deposition together explained 90% of the variance amongst individuals in observed rates of growth. The results support the hypothesis that individual variability in the energy costs of protein turnover and in the efficiency of protein deposition during rapid growth are significant factors providing a link between individual genotype and its phenotypic expression as growth.