Automatic feature tracking on two Landsat images (acquired in January 2000 and December 2001) generates a complete velocity field of Mertz Glacier, East Antarctica. This velocity field shows two main tributaries to the ice stream. Between the tributaries, a likely obstruction feature in the bedrock results in a slow-down of the flow. A third Landsat image, acquired in 1989 and combined with the 2000 image, permits the determination of the glacier mean velocity during the 1990s. Although some parts of the Mertz Glacier system show evidence of slight speed increase, we conclude that the Mertz flow speed is constant within our uncertainty (35 m a-1). Using this complete velocity field, new estimates of the ice discharge flux, 17.8 km**3/a (16.4 Gt/a), and of the basal melting of the tongue, 11m/a of ice, are given. Our results lead to an apparent imbalance of the drainage basin (ice discharge 3.5 km**3/a lower than the accumulation). Considering previous studies in the Mertz Glacier area, we discuss the uncertainty of this imbalance and the problems with accumulation mapping for this region.