Four breeding lines of Sydney rock oysters, Saccostrea commercialis, progeny of parents selected for faster growth for one generation, were compared in a 17-month experiment with two control lines (spat produced from non-selected oysters). Oysters from two out of four selection lines were significantly heavier (P < 0.05) than the controls. The improvements in weight gain for the two selection lines grown loose on trays were 2.9 and 8.5%, and for the two slat lines (grown fixed on PVC slats) they were 0.0 and 5.0% for the first generation of selection. These results provide encouragement for the selective breeding of Sydney rock oysters for weight gain.