We use atomic force microscopy to explore the mechanisms of crystal growth of the protein lumazine synthase. We find that at moderate supersaturations, the only mechanism of layer generation on the (0 0 1) face is via landing of ∼100 nm large dense objects. After landing on the crystal surface, the objects flatten and yield stacks of steps. These steps merge continuously with each other and with the underlying lattice. These observations allow us to conclude that the dense objects are not microcrystals nucleated in the solution, but are droplets of the dense liquid phase. Dynamic light scattering revealed the presence of dense liquid droplets, which are metastable with respect to the solution, have a finite lifetime of several seconds, radii of 50–200 nm, and consist of 10 2–10 4 molecules. Under the influence of the periodic field of the crystal upon landing on the crystal surface, these droplets stabilize and transform into stacks of crystalline layers, whose spreading results in crystal growth. Since crystals of this protein do not have dislocations, and two-dimensional (2D) nucleation of new layers does not occur, likely because of high barriers, the sedimentation and structuring of metastable droplets of protein-rich liquid is the only mechanism providing for crystal growth.