The study objective was to clarify how the growth stages of the Pacific oyster Crassostrea gigas affect selective suspension-feeding of particulate organic matter (POM) and the composition of biodeposits. A day-long (22 h), continuous-flow mesocosm experiment was conducted with 3, 15, and 27 mo old oysters. The suspended particulate matter (PM), settled PM (mostly biodeposits in the oyster mesocosms), and oyster soft tissues were analysed to determine the content of fatty acids, organic carbon, and nitrogen, as well as the carbon and nitrogen stable isotope ratios to trace compositional changes in POM through oyster biodeposition. Regardless of oyster age, the stable isotope ratios of biodeposits were similar to those of the body tissues but not to those of the suspended PM, indicating that oysters selectively fed on assimilable fractions of POM. Compared with the suspended PM, a higher concentration of long-chain polyunsaturated fatty acids was found in the body tissues and, consequently, in the biodeposits; in contrast, the concentrations of shorter-chain fatty acids were generally lower in the biodeposits. Furthermore, the biodeposits produced by the older oysters had higher carbon, nitrogen, and fatty acid contents compared with the biodeposits produced by the 3 mo old oysters. The oxygen consumption rate of biodeposits was positively related to organic carbon content, but less so to fatty acid composition. Our findings demonstrate that older oysters not only produce larger amounts of biodeposits, but that these biodeposits have higher organic and fatty acid contents, potentially exhibiting greater effects on biogeochemical and ecological processes in nearby benthic habitats.