Two size grades (13.5 and 18.2 mm average shell length spat −1) of juvenile Sydney rock oysters, ( Saccostrea commercialis Iredale & Roughley) were grown at a range of stocking densities (0.5 to 6.0 l of oysters cylinder −1) in PVC cylinders for 3 months. Cylinders were deployed on horizontal hardwood frames on an intertidal lease, where they rotated in response to changes in tidal level. For each grade, spat mortality was similar ( P > 0.05) for stocking densities of 0.5 to 4.0 l cylinder −1 (mean ± s.e. 11.7 ± 1.1% and 22.5±2.3%, respectively) but higher for the larger grade (33.3 ± 1.7%; P < 0.05) at the highest stocking density. For both grades, weight gain and length increase declined as stocking density increased ( P < 0.001), although differences for initial densities above 3.0 and 4.0 l of oysters cylinder −1, for the smaller and larger grades respectively, were not significant ( P > 0.05). Biomass gain and volume of oysters both increased with increasing stocking density ( P < 0.05), although increases above initial densities of 2.0 and 3.0 l of oysters cylinder −1, for the smaller and larger grades, respectively, were not significant ( P > 0.05). For both grades, coefficient of variation for weight gain and shell length increase data increased ( P < 0.001) with density. For maximum growth and minimum coefficient of variation for weight gain and shell length increase, 0.2 and 0.4 g spat should be stocked at low densities of 0.5 or 1.0 l cylinder −1. To optimise biomass gain, while minimising growth reductions and size variation, 0.2 and 0.4 g spat should be stocked at densities of 2.0 and 3.0 l of oysters cylinder −1, respectively.