Controlled reproduction of the European flat oyster requires the development of tools adapted to this species, including a practical anaesthesia and gonad sampling protocol to facilitate sex determination and the verification of gametogenesis. Three replicate groups of 10 oysters (mean weight +/- SD: 29.9 +/- 8.5 g) were anaesthetised in 5 L containers using magnesium chloride designed either for laboratory (Flucka (R)) or agricultural (Dead Sea Work (R): DSW) use, at two concentrations (50 or 72 g L-1). No significant differences were observed in the percentages of oysters anaesthetised or subsequent oyster mortality with the different anaesthetics or concentrations, but increasing water temperature from 14.9 to 18.8 degrees C significantly increased the number of oysters anaesthetised after 3 h. Increasing anaesthesia duration from 1 to 22 h significantly increased the percentage of oysters anaesthetised but did not affect subsequent oyster mortality. Gonad sampling of anaesthetised oysters did not increase oyster mortality either. A reliable anaesthesia protocol was, therefore, defined using 50 g L-1 DSW (R) magnesium chloride for a 2 to 3 h duration. This protocol was validated by monthly anaesthesia and gonad sampling on the same oysters over a three month period, during which a percentage of 95 +/- 2% anaesthetised oysters was observed. Compared with controls (oysters that were neither anaesthetised nor sampled), oyster mortality of monthly anaesthetised batches showed no significant increase. (C) 2010 Elsevier B.V. All rights reserved.