The study measured the mean and ranges of seasonal changes in whole body energetic values in two age groups of the Sydney rock oyster Saccostrea glomerata from the Clyde River, NSW, Australia. Oysters were collected over four seasons from two aquaculture-relevant age groups (24 and 36 mo) identified as key harvesting ages. Protein, lipid, glycogen energy equivalents, and electron transport system (ETS) activity were determined to identify seasonal changes in energy stores and consumption and to calculate cellular energy allocation. There were clear seasonal trends in energy changes, with glycogen and lipid energy equivalents highest in summer, decreasing to their lowest concentrations in autumn, and then slowly increasing through winter and into spring. Protein energy equivalents and ETS activity demonstrated an opposite trend, with increases in autumn and stable through the other seasons. Cellular energy allocation showed seasonal change, with highest values over summer, gradually decreasing in autumn and winter, and then starting to increase again in spring. These results indicate that seasonal changes in temperature, salinity, and food availability as well as reproductive cycles have a significant influence on S. glomerata energy storage and use. Developing an understanding of energetic responses in bivalves in response to environmental conditions can facilitate a more detailed understanding of growth potential and responses to aquaculture practices such as stocking density. As well, understanding natural energetic fluctuations allows a baseline for the assessment of other field-based stress responses such as anthropogenic contamination in S. glomerata.