The construction of a rock and earth-filled dam in the La Grande River valley, Quebec has created a hydroelectric reservoir with a maximum depth of 145 m and a volume of 61.7 × 109 m3. No seismicity in this Precambrian Shield region above magnitude 0.1 was observed in the reservoir zone in the 27 months preceding the filling. Microearthquakes of magnitude less than 1 started under the reservoir when a water depth of 90 m was reached after 30 days and this activity continued for about 3 months. A second series of microearthquakes began in the same active zone when the rate of filling increased twofold. The seismically active area is confined to a zone 4 km × 7 km and no deeper than 5 km. The microearthquakes are considered to be the result of minor reactivation of old faults under the reservoir in response to the increased pore pressure.