The Sydney rock oyster, Saccostrea glomerata, mass selection breeding program was established in 1990 in the Georges River, New South Wales, and has successfully developed lines of oysters resistant to QX disease (Marteilia sydneyi) and winter mortality disease (WM; Bonamia roughleyi). Each new generation is assessed in the Georges River, but the performance of these oysters in other estuaries affected by these diseases is unknown. To investigate the potential for genotype and environment interactions, and to assure farmers, survival and growth of progeny of the fourth-generation breeding lines developed in Georges River were assessed in two other key farming areas; the Hawkesbury River and Merimbula Lake, NSW, Australia. Hawkesbury River is affected by QX and was used to assess the QX-resistant oysters (QXr). Merimbula Lake is affected by WM, and this estuary was used to assess the WM-resistant oysters (WMr) and the dual WM and QX resistant line (WM QXr). Oysters that are QXr in the Hawkesbury River, and the WMr oysters in Merimbula Lake, had significantly lower mortality and faster growth compared with controls at the final measurement. Oysters that are QXr in the Hawkesbury River at the Marra Marra Creek and Kimmerikong Bay sites attained the 50-g "plate grade" benchmark after 24 mo of culture and had lost only 29.4% and 16.9% of oysters originally stocked, respectively. Meanwhile, QX caused losses of 93.7% and 95.7% in controls at these 2 sites, respectively. In Merimbula Lake, at the conclusion of the experiment WMr oysters were significantly heavier, were larger, and had significantly lower mortality than WM QXr and control oysters. Growth and survival of WMr, WM QXr, and QXr oysters were comparable with that measured in the Georges River.