An expanding spread seismic reflection survey has been conducted across the Snake Bay–Kakagi Lake greenstone belt in northwestern Ontario. Receiver and shot arrays with multiple shots per location helped to maintain a high signal to noise ratio in most of the data. Distances between the shots and receivers ranged from 1.04–8.48 km and the total charge per shot location varied from 26–86 kg. After computer processing the data, numerous coherent reflections were observed from near vertical and near horizontal discontinuities.Prominent early reflections were used to map a granite–greenstone contact to the south of the profile and a section of the Long Bay fault zone to the northeast of the profile. A noticeable absence of reflections from the Aulneau granite batholith–greenstone contact suggests that this contact dips westwards, towards the centre of the batholith.From the later reflections a model of the deep crust beneath the Snake Bay–Kakagi Lake greenstone belt was derived. This model, which represents a lateral extension of the Aulneau crustal model, consists of a three-layered crust. The top crustal layer is 19 km thick with Pg and Sg velocities of 6.2 and 3.5 km/s respectively, the middle layer is 3 km thick, and the lower layer extends to the Mohorovicic discontinuity at 38 km depth.