The western part of the Bothnian Bay (northern Gulf of Bothnia) has been investigated by means of seismic reflection and refraction measurements. The sea floor was found to consist largely of glacial drift. The postglacial sediments are unevenly distributed with a dominance in the deeper basins. Till accumulations form an abundance of distinctly separated irregular beds. The upper till beds are provisionally attributed to the Weichselian glaciation whereas the lower beds are regarded as pre-Weichselian. The crystalline bedrock of the Bothnian Bay is to a great extent covered by sedimentary rocks of the Upper Riphean Muhos formation. In the central parts of the Bay, the Muhos rocks are succeeded by almost 100 m of sedimentary rocks which structurally closely resemble the Lower Cambrian stratal sequence of the Bothnian Sea.