Gravity surveys of the Timmins–Senneterre mining belt of northeastern Ontario and western Quebec were made by the Dominion Observatory during the period 1946–1964. The Round Lake batholith is one of several composite granitic plutons of the Algoman series which are outlined by intense negative gravity anomalies. The anomaly over the Round Lake batholith can be explained by the large density contrast (0.22 g/cm3) between the granite and surrounding Keewatin volcanic rocks.Two possible models of the batholith are presented which depend on different assumptions as to the composition of the upper crust. The first model involves normal faulting of the batholith to explain the variations in anomaly level within the batholith. In this model the granite is assumed to be homogeneous in density and extends to a maximum depth of 10 km. Alternatively density variations corresponding to a facies change within the pluton may be the major cause of the local internal anomaly variations. In this interpretation the true thickness of the granite cannot be evaluated as the whole region is assumed to be underlain by granite, but the maximum thickness of the surrounding basic volcanic rocks is 5 km.