The Britt Domain in the southwestern Grenville Province of the Canadian Shield is believed to be an exposure of high-grade (upper amphibolite facies) mid-crustal rocks of predominantly granitic and granodioritic composition. A 270 km refraction line was conducted by Lithoprobe across the Britt Domain and the Grenville Front Tectonic Zone to the north in order to determine the deep velocity structure under the region. This data set demonstrates a uniform velocity structure to a depth of 15 km in the central Britt Domain, with near-surface P- and S-wave velocities of 6.15 and 3.55 km/s, respectively, and linear vertical gradients at depth of 0.02 and 0.01 s−1, respectively. The data also show that the Grenville Front Tectonic Zone is strongly anisotropic at shallow depths. To determine the acoustic properties of rocks from the Britt Domain, 80 velocity samples representing different lithologies in this area were measured at confining pressure up to 600 MPa. These studies show that P-wave velocities at 600 MPa range from 6.29 km/s (granitic gneiss) through 6.51 km/s (intermediate gneiss) to 6.90 km/s (mafic rocks), and have an area-weighted mean of 6.36 ± 0.08 km/s. S-wave velocities for selected samples range from 3.54 km/s (paragneiss) through 3.62 km/s (granitic gneiss) to 4.04 km/s (amphibolite). P-wave velocity anisotropy is weak in the granitic rocks (1.1% on average), but stronger in paragneisses and amphibolites from the region (5.2–5.9%). Comparison of laboratory and refraction data suggests that the Britt Domain is granodioritic in composition to mid-crustal levels. The lower crust beneath the Grenville Front Tectonic Zone in the central Britt Domain appears to be composed of diorite at the top, but becomes increasingly mafic toward the Moho.