Microdeformation in thin films of rubber modified poly(methyl methacrylate) has been investigated using TEM. The effects of two-layer core-shell particles with rubbery cores, and three-layer particles with glassy cores are compared. While the former undergo relatively coarse cavitation and may act to block fibril drawing in matrix crazes, the rubber shell of the three-layer particles tends to fibrillate, and they are readily incorporated in the crazes.