The crystalline lens of the vertebrate eye grows throughout life. This growth may be enormous in fish, while the lens must be functional from larva to adult. During growth, the fiber cells of the lens must increase the concentration of specific proteins (crystallins) in the cytoplasm to increase refractive index. However, the bulk of the fiber cells in a vertebrate lens are denucleated and have no organelles to synthesize proteins. To study how this problem is solved, we first measured lens fiber cell thickness in the Nile tilapia, a teleost fish. In the lenses from 25 fish, in two size groups, fibers were considerably thinner than in other vertebrates. Fiber thickness was about constant along the radius of the lens and the same between the size groups. Since our results provided no evidence for shrinkage of lens fiber cells with growth (expected if protein concentration is increased by expelling water) we included eight additional teleost species to elucidate the mechanism by which the cells increase crystallin concentration. In all species, fiber cell thickness was about constant throughout the lens, with species-specific values. The changes in fiber cell thickness expected from an increase in crystallin concentration by removal of water were modeled. Shrinkage in cell thickness by up to 66% would have been necessary to reach the required crystallin concentration. We conclude that crystallin concentration in denucleated lens fiber cells is increased by transport of proteins from synthetically competent cells in the periphery of the lens.