Efficiencies and photofractions for monoenergetic gamma rays, from a broad parallel beam, incident on sodium‐iodide crystals of various sizes have been calculated by the Monte Carlo method. The calculations were carried out for photon energies of 0.279, 0.661, 1.33, 2.62, and 4.45 Mev; the crystals considered were right circular cylinders with radii of 1.0, 2.0, 4.0, 8.0, 12.0, and 16.0 in. and, for each radius, crystals of heights 1.0, 2.0, 4.0, and 8.0 in. The effect of the escape of annihilation gammas from the crystal was taken into account in the Monte Carlo calculation; however no correction was made for escape of bremsstrahlung. This correction is important only at the largest energies and for the smallest crystals where it affects the photofraction by a few percent.