We present measurements of ice thickness, gravimetry and surface elevation on Pine Island Glacier, West Antarctica, separated by a period of 49 years. At one station, on the main trunk of the glacier we measured a surface elevation lowering with no significant change in ice thickness. We interpret these as indicating subglacial erosion of 31.8 {plus minus} 13.4 m at this location, at a mean rate over the measurement period of 0.6 {plus minus} 0.3 m a-1, and suggest that a current erosion rate of ~1 m a-1 is possible. Our results emphasize that locally, basal processes can have a significant effect on ice sheet changes, particularly where fast-flowing ice has an easily erodible bed.