A sub-critical seismic reflection survey has been conducted across the Aulneau granite batholith in northwest Ontario. Shot to receiver distances ranged from 2.23 to 71.9 km and the total amount of explosives used for each shot–receiver configuration ranged from 54 to 327 kg. Multiple shots with vertical stacking, frequency filtering and time varying velocity filtering were used to increase the signal to noise ratio for each trace. Reflections from shallow vertical faults and deep horizontal crustal discontinuities were observed on the processed records. From these data a model of the crustal structure in the region of the Aulneau batholith has been derived. The model has a number of near vertical faults and an essentially three layered crust in which the upper layer is 16 km thick with P-wave and S-wave velocities of 6.1 and 3.6 km/s, respectively, in the top part of the layer. The average P-wave velocity throughout the layer is 6.2 km/s. This layer probably represents the relatively homogeneous part of the batholith. The middle layer is 5 km thick beneath the shot point and thins to the southeast. It has a P-wave velocity of 6.9 km/s. The lower layer extends to the Mohorovicic discontinuity at 38 km depth and has an interval velocity of 7.2 km/s. The proposed crustal model is generally supported by other geophysical data collected in northwest Ontario.