Free-air gravity anomalies near Nias Island include a -100 mGal low centered 10-20 km land-ward of the trench axis and a +80 mGal high over the outer arc rdige but also show a few large anomalies unrelated to topography. A -90 mGal low is found on the structral high between the Banyak Islands and Sumatra, north of Nias. An 80 mGal rise (at a gradient of 8 mGal/km) is modeled by a near-surface body of high-density material (oceanic crust?) 4 km wide and tens of kilometers long. This slab may be exposed on the southwest coast of Nias, where ultramafic bodies have been mapped, and its northerly trend (compared with northwesterly bathymetric trends) may be related to other obliquely trending structures near here, to a proposed strike slip fault which crosses Nias, and/or to the basement high topped by the Batu Islands. A -30 mGal free-air low over the forearc basin can be modeled best if pre-Miocene m¿lange or continental crust underlies the basin, and it seems unlikely that the basin can be underlain by a large thickness of oceanic crust.