Twenty-one new heat flow measurements in the Mariana Trough delineate a central zone of high heat flow corresponding to an axial topographic high and a flank zone of low heat flow. The high heat flow is an indication that the axial high is a narrow zone of magmatic intrusion and that sea floor spreading is occurring in the Mariana Trough in a manner similar to its occurrence on midocean ridges. The low heat flow values appear to be the result of the occurrence of hydrothermal circulation in the oceanic crust of the Mariana Marginal Basin.