Maps of the earth's total magnetic intensity at a 100-gamma contour interval are presented for most of the Gulf of Mexico and about 12,600 square miles surrounding Caryn Peak in the northwestern Atlantic. These were prepared mainly from data obtained during VEMA cruise No. 3 early in 1954. The field is remarkably uniform over an area including the deep basin of the Gulf of Mexico and extending northward almost to Texas and Louisiana. Numerous essentially circular anomalies of several hundred gammas amplitude exist near the edges of the calcareous West Florida and Campeche Banks. Caryn Peak exhibits a similar anomaly which is attributed to remanent magnetization of basic igneous rock. It is concluded that at Caryn Peak during the volcanic activity, probably in late Cretaceous time, the earth's magnetic field had approximately the same direction as now.It is concluded that the scarps bounding the calcareous banks are not tectonic. It is suggested that they roughly coincide with lines of now buried basic volcanoes around which the limestone banks developed. In the area of the Gulf of Mexico which has been surveyed there is no anomaly like the several hundred gamma linear positive found off the eastern coast of North America, near the transition from a thick continental to a thin oceanic crust. If present, it must lie near the shoreline in the vicinity of Texas and Louisiana. This supports other evidence that the crust beneath the outer shelf and continental rise off Texas and Louisiana is of a thin oceanic type similar to that under the main basin. In this interpretation a thick column of sediments has spread out over part of the deep basin.