In this paper, we propose a seamless switching scheme for scalable video bitstreams that fully takes advantage of both the high coding efficiency of non-scalable bitstreams and the flexibility of scalable bitstreams. Small bandwidth fluctuations are accommodated by the scalability of the bitstreams, while large bandwidth fluctuations are tolerated by switching between scalable bitstreams. The major contribution of this paper is a flexible and effective scheme for seamless switching between scalable bitstreams that significantly improves the efficiency of scalable video coding over a broad bit rate range. When the channel bandwidth drops below the effective range of a scalable bitstream operated at higher rates, the proposed scheme can switch at any frame from the current scalable bitstream to one operated at lower rates without sending any overhead bits. Additional bits are only necessary when switching from a scalable bitstream operated at lower rates to one operated at higher rates. Experimental results show that the proposed scheme significantly outperforms both the approach with a single scalable bitstream and the approach of switching among multiple non-scalable bitstreams.