The tuffaceous sandstones and siltstones of the Ola Bula Formation in central Flores. east Indonesia, contain many fossil sites. Here, excavations at Boa Lesa and Dozu Dhalu and the results of regional site surveys are described. Stone artefacts indicate that hominids had arrived on the island by 840,000 years ago, post-dating a major change in the Lower Pleistocene fauna. Since water crossings were required to reach Flores from mainland Southeast Asia, this evidence has implications for the intellectual, technological and linguistic capabilities of early hominids.