In this article, we provide evidence that a frequent source of diversity between mammalian transcripts occurs as a consequence of species-specific alternative splicing (AS) of conserved exons. Using a highly predictive computational method, we estimate that >11% of human and mouse cassette alternative exons undergo skipping in one species but constitutively splicing in the other. These species-specific AS events are predicted to modify conserved domains in proteins more frequently than other classes of AS events. The results thus provide evidence that species-specific AS of conserved exons constitutes an additional potential source of complexity and species-specific differences between mammals.