The complexity of the relationships between Alexandrium minutum (Am.) concentration in the water ([A.m.](w)), Paralytic Shellfish Poisoning contamination in the digestive gland ([PSP](dg)) and valve behavior was explored in oysters Crassostrea gigas. Two experiments were conducted, during which oysters' valve behaviour were analyzed. Oysters, first acclimated for 10-days with the non harmful microalgae Heterocapsa triquetra (H.t.), were exposed to four microalgae mixtures at constant total concentrations of 10 x 10(3) cells ml(-1) (experiment-1) and 5 x 10(3) cells ml(-1) (experiment-2): 100% Am.; 50% A.m.-50% H.t.; 25% A.m.-75% H.t.; 100% H.t. At the end of experiment-2, [PSP](dg) were measured. At 10 x 103 cells ml(-1), the microalgal ingestion decreased (p < 0.05) with increasing [A.m.](w) but not at 5 x 10(3) cells ml(-1) (p > 0.05). The frequency of microclosures specifically increased with [A.m.](w) (p < 0.05) and the opening duration with [PSP](dg) (p < 0.0001). Oysters exhibiting the maximum increase in opening duration also exhibited the highest [PSP](dg). The results are discussed in terms of oyster physiology and origin of the behavioral response. (C) 2011 Elsevier Ltd. All rights reserved.