A long refraction line was established between the Lizard and Start peninsulas during a large-scale seismic experiment carried out in SW England during 1979. Interpretation of first arrival data indicates a shallow crustal structure consisting of two layers of regional extent. The upper layer of average velocity 5.15 km/s is interpreted as Devonian metasediments. The underlying layer with a velocity of about 6.10 km/s lies at a depth of several kilometres beneath the Lizard complex but rises eastwards to occur at or near surface in the Eddystone and Start areas. The lower layer is believed to represent a pre-Devonian crystalline basement of which the metamorphic rocks of Start and Eddystone are constituent parts. By contrast, the Lizard complex is interpreted as a thin surface sheet overlying Devonian metasediments.