Radiocarbon dates, continuous seismic reflection lines, echograms, bottom sediments, and surface topography are used to establish the geomorphological development and post-Pleistocene sea level changes in Northumberland Strait. The oldest recognizable topography is a pre-Pleistocene drainage system eroded into the bedrock. During the post-Pleistocene marine transgression four terraces were formed. Their ages and spatial distribution reflect the combined effect of postglacial sea level changes and crustal movement. After glacial withdrawal the strait was occupied by two estuaries separated by an isthmus. In the west, the sea initially covered much of the present land which had not yet recovered from glacial depression. Between about 13 000 and 7000 years B.P. glacial rebound caused temporary regression of the shoreline until rise in sea level again became the dominant force. In the east, rise in sea level always kept pace with any rebound, and has gradually enlarged and deepened the submerged area. Northumberland Strait became a continuous body of water about 5000 years B.P. when the isthmus to Prince Edward Island was breached. Comparison with sea level curves from other areas of the world indicate that Northumberland Strait has been subsiding during the last 7000 years. The rate of subsidence appears to have been greater in the western part of the strait than in the eastern. It is suggested that this part of the Maritime Provinces lies between areas of positive and negative crustal movement.