A new crystalline form, thin laminar microcrystals of 70 S ribosomes from Thermus thermophilus, have been obtained. Negatively stained ordered sheets derived from these crystals were studied by electron microscopy with subsequent computer image processing. The sheets have two-dimensional unit cell parameters a= b=480 A, y=90" and a resolution limit of about 30 A. The microcrystals are suitable for three-dimensional image reconstruction.