Three magnetic classes of ocean basalts are defined on the basis of thermomagnetic and microscopic analysis. Moderate heating of Class I basalts results in an anomalous increase in Curie point, magnetization intensity, the ratio TRM to NRM, and magnetic susceptibility. The same thermal treatment of Class II rocks results in an anomalous decrease in all magnetic parameters except the Curie point. Class III rocks are stable. Several Class I and II ocean basalts possess the property of self reversal. Class I and II rocks contain metastable magnetic minerals and readily transform into stable Class III rocks when heated. The transformation mechanisms are described and the characteristics of each class are tabled. It is concluded that in situ cores are necessary to discover the composition of the ocean crust, and the distribution of the different magnetic classes of ocean basalts within the ocean crust. Otherwise, little faith can be placed in any interpretation of ocean magnetic anomalies.