Protein crystals form in supersaturated solutions via a nucleation and growth mechanism. The amyloid fibrils of denatured proteins also form via a nucleation and growth mechanism. This similarity suggests that, although protein crystals and amyloid fibrils are distinct in their morphologies, both processes can be controlled in a similar manner. We show that ultrasonication is one of the most effective methods of agitation for accelerating spontaneous fibrillation. We also show that ultrasonication is also useful for forming crystals of proteins. We address the underlying mechanism of the ultrasonication-dependent breakdown of supersaturation, leading to the formation of amyloid fibrils or protein crystals.