The performance of file transfer applications running over an ATM infrastructure investigated, taking into account both the impact of the ATM and AAL layer protocols and the features of the adopted transport protocol. Two transport protocols in particular are considered: TCP and XTP. The two transport protocols have quite different characteristics: while TCP is a window-based protocol, XTP is rate-based; while TCP contains algorithms for congestion avoidance, XTP includes no such feature. The performance investigation was performed by simulation, using CLASS, a simulation tool for the study of ATM networks at the cell and burst levels.