A UDP-based application-level file transfer protocol, UFTP, with flow control using fuzzy logic is proposed in this paper. UFTP is designed for moving large-size files over WANs where traditional file transfer protocols are found to be very inefficient. Compared to FTP, which uses TCP as its transport-level protocol, it uses UDP packets to send data to avoid round trip propagation delay, a limiting factor in data transferring. The fuzzy flow control (FFC) module calculates the date flow rate for UFTP by monitoring the packet receive success rate and its rate of change, then by using a set of fuzzy linguistic rules. The design scheme of UFTP with FFC is explained in details and the performance of the FFC scheme is evaluated through experiments in a high-speed WAN environment. The experimental results show that UFTP, with flow control using FFC, is up to 7.4 times faster than the traditional protocol FTP. It is also shown that FFC offers fast transient response and leads to low end-to-end transmission delay.