We describe the valve-activity behavior of oysters, Crassostrea gigas, exposed experimentally to the harmful alga Alexandrium minutum (≈ 3500 cell ml− 1) for 7-day periods under laboratory conditions. Our aim was to assess behavioral responses of oyster species during a mimicked bloom exposure. We determined different characteristic parameters of valve activity, such as daily valve opening duration, daily number of micro-closures, and valve-opening amplitude using a High Frequency-Non Invasive valvometer. In comparison with oysters exposed to non-toxic algae, T-Isochrysis or Heterocapsa triquetra, the valve activity of C. gigas is measurably different when exposed to toxic algae A. minutum. Surprisingly, daily valve-opening duration increased, as well as micro-closure activity, while valve-opening amplitude decreased. The response to A. minutum is fast, within 1 h after algae exposure. Following A. minutum exposure, recovery to control patterns was observed within 4-5 days. The behavioral alterations upon exposure to A. minutum can be thus used as a complementary physiological variable to other well-established physiological and biochemical measurements.