Two-ship seismic refraction profiles were made across the Sunda Arc south from central Java and Bali; in the Flores. Banda; and Arafura seas; and in the Timor Trough, Sourth of the trench off Java and Bali, the crust is oceanic. Beneath the trench and outer nonvolenic ridge the mantle and crust are depressed and dip northward, some thickening occurring the oceanic crustal layer. The outer ridge, along the gravity minimum, consists primarily of material having velocities typical of sedimentary rocks, and a structure of north-dipping imbricate thrust sheets is suggested. Layer 2 jumps upward about 5 km under the crest of the ridge. From this point to the islands the crust is probably oceanic but intermediate in thickness, with mantle arrivals in all stations and oceanic crust arrivals in the stations closet to the islands. We interpret this as thickened old oceanic crust and mantle trapped here by a seaward jump in the subduction zone, probably in early Tertiary time. North and east of Bali, behind the volcanic arc, the crustal structure is intermediate between oceanic and continental. Farther to the east in the Flores Basin the thickness decreases, a finding suggesting that this is the transitional edge of cratonizatin of Sunda Shelf, and typical thin oceanic crust is found farther to the east in the sourthern Banda Sea. The Arafura Sea is underlain by continental crust which continues at least to the deepest part of the Timor Trough.