[en] It is shown that a tokamak with a major radius larger than ∼6 m and a toroidal field of 10 T can reach ignition by ohmic heating alone at a relatively low peak density [n(o)∼1x10²⁰ m^-3] even with confinement degradation due to alpha-particle heating, provided a confinement enhancement factor of γ_H = 2 over the Goldston scaling is assumed. The critical toroidal field and plasma current required for ohmic ignition have been estimated for various sizes of tokamaks with major radii R = 2 m (compact), 6 m [International Thermonuclear Experimental Reactor (ITER) class], and 10 m (large tokamaks). If a broad current profile can be achieved transiently, the critical toroidal field and plasma current can be further reduced by the enhancement in the ohmic heating power. 37 refs., 7 figs