Cavitation formation in a high strain rate superplastic Al-Mg-Si composite, reinforced with 20 vol.% Si3N4 particulates, was observed by a high resolution scanning electron microscope. The composites were pulled by the tension test under steady true stress at an optimum superplastic deformation condition of 8 MPa and 833 K. Many cavities were formed at/near the interfaces between the matrix and particulates. The number of cavities increased slowly and the growth was so sluggish with increasing strain. The size of most cavities, up to strain of 1.0 at least, were the order of the particulate sizes at nearly 0.5 Irm, whereas above strain of 1.4 a few large cavities almost equal to grain sizes appeared by growth and/or interlinkage of the cavities. The large cavities near the highly deformed tip of the failed specimens rapidly grew up and made alignments along the tensile axis.