Efficient implementation of TCP for the Internet requires a precise determination of congestion window by the source TCP agent. This paper proposes a fuzzy implementation of TCP (Fuzzy TCP), instead of current slow-start/congestion avoidance approach for efficiently determining size of the congestion window at each time a new acknowledgement receives. The scheme uses the current size of congestion window and the slow-start threshold (ssthresh) as well as the last and previous round trip times. The proposed fuzzy controller updates the size of congestion window using a fuzzy system. The approach is applied to the source agent and the destination TCP remains unchanged.