Preliminary results of gravity observations on the Southeast Arm of Sulawesi in eastern Indonesia provide evidence on the structure of the large ultramafic belt and associated melange units. Gravity highs at the schist-ultramafic contact indicate that the ultramafic rocks are very thick there, possibly dipping beneath the schist. The gravity effect of the ultramafic rocks decreases eastward away from the schist, and a gravity low over a large zone of limestone and melange requires a thick body of low density material, probably melange, to nearly surround the limestone blocks. The interpreted structure imposes constraints on models of emplacement of the ultramafic slab and is compatible with slicing off slivers of ocean crust and upper mantle in a subduction zone, but it is not compatible with obduction or with diapiric intrusion. Uplift of ultramafic rocks was aided initially by the presence of abundant low-density melange and, finally, by the collision of Sulawesi with the Banggai-Sula Islands.