In a DEPleted Field Effect Transistor (DEPFET) sensor a MOSFET is integrated on a sidewards depleted p-on-n silicon detector, thereby combining the advantages of a fully depleted silicon sensor with in-pixel amplification. A 450 μm thick DEPFET was tested in a testbeam. The S/ N was found to be larger than 110. The position resolution is better than 5 μm. At a seed cut of 7 σ, the efficiency and purity are both close to 100%. In the readout chip a zero-suppression capability is implemented. The functionality was demonstrated using a radio-active source. The predicted impact parameter resolution of a 50 μm thick DEPFET vertex detector, is much better than required for the International Linear Collider (ILC).