Aquatic toxicity assessments based on inhibition of algal growth have been performed, as described in the French standardized procedure T90-304. The cellular growth was monitored by acoustic micro-sensors (quartz thickness shear mode) instead of the usual optical techniques. These experiments have shown that the real component of the interfacial acoustic impedance, measured by the sensor, displayed a large linear variation along the 3-day algal growth. The acoustic technique appears to have an interesting potential for conducting automatic algal bioassays.