The energy content and biochemical composition of seed of the mussel Mytilus galloprovincialis of two distinct origins (subtidal and rocky shore) were compared after both groups were transferred to the water column (subtidal habitat) in the Arousa Ria, NW Spain. The experimental period comprised a total of 228 days. The relative percentages of carbohydrates, glycogen and lipids were significantly higher in the subtidal mussels in the first 15 days, whereas the proteins were significantly higher in the rocky shore mussels during the first 36 days. A multiple regression analysis referred to the same time period showed that the origin of the mussel seed influenced significantly the variance observed in the energy values of all biochemical components (measured in terms of kJ mg–1). In the period between days 50 and 110, mussel origin only showed a significant effect on the lipid energy content, but in contrast to the previous period, the content was greater in the rocky shore mussels. The multiple regression analysis showed that after 125 days of culture, mussel origin did not influence the variance observed in the energy values of all biochemical components, whereas the ratio chlorophyll a/particulate organic material, temperature and total particulate material influenced significantly the variance observed in the different energy values of all biochemical components. Changes observed in the energy content of carbohydrates and lipids throughout the different periods of this study can be related to catabolismanabolism processes linked mainly to the availability of phytoplanktonic food. Moreover, with regard to lipids, our results suggest that changes were also associated with reproductive activity.