Winona basin is a deep water sedimentary basin forming part of Explorer plate and located off the northern end of Vancouver Island, British Columbia. During 1975 a deep seismic sounding survey, using explosive charges and multiple hydrophones as receivers, was carried out over the basin. Three reversed refraction profiles were shot, one along and two across the basin. A preliminary look at data along the basin indicated reasonable lateral homogeneity so interpretation was based on both travel time and amplitude characteristics through the use of synthetic seismograms. However, the two profiles across the basin indicated significant lateral variations in crustal structure, so a ray tracing approach was used. The sediments of Winona basin have a maximum thickness of approximately 5 km, consisting of ~2 km of low-velocity (average ~2.5 km/s) layers and ~3 km of higher-velocity (~4.3 km/s) material. Beneath the sediments there is a rapid increase in velocity to ~5.4 km/s, the top of the oceanic crust. Below this and along the basin, the interpretation indicates three zones with thicknesses of 1 to 3 km and having low-velocity gradients; these being separated by transition zones with depth intervals less than 1 km and having high-velocity gradients. The three zones with low-velocity gradients are represented approximately by 1 km at 5.4 km/s, 2.7 km at 6.2 km/s, and 2.5 km at 6.7 km/s. With the inclusion of the zones of high-velocity gradient, an average subsedimentary crustal velocity of ~6.3 km/s and thickness of ~8 km are obtained. The upper mantle velocity is ~7.8 km/s. Across the basin, the crustal model shows layers dipping eastward from the westward side, forming a bowllike structure. Two-dimensional gravity calculations based on the seismic model are compatible with a gravity profile derived from a free air anomaly map. The thick crust and structural variations are consistent with, but do not provide direct evidence for, the complex plate interactions embodied in the tectonic reconstructions proposed by other scientists.