The results of underwater gravity measurements of a reconnaissance nature in Hudson Bay and of regional gravity surveys over adjoining regions of the Canadian Shield and Hudson Bay Lowlands are presented in the form of a Bouguer anomaly map. The general anomaly pattern is discussed and an interpretation is presented. Analysis of anomaly profiles crossing Hudson Bay using seismic data for control, shows (1) that Paleozoic and recent sediments have little overall effect on the gravity field, and (2) that variations in depth to the crust-mantle boundary have little or no correspondence with variations in the gravity field. It is believed that the gravity variations are largely due to systematic density differences in the Precambrian basement rocks. An important conclusion is that the more positive gravity field over the northern part of Hudson Bay marks a major change in structure and that the Churchill crustal block does not extend uninterruptedly eastward; it is suggested that the positive anomalies reflect high-density metamorphic gneiss or basic volcanic rocks underlying the sedimentary veneer. Evidence is presented which strengthens the hypothesis that the Kapuskasing gravity anomaly and its northern and southern extensions mark an ancient zone of crustal rifting of continental proportion.