  We investigate relation between Dehn fillings and commensurability of
hyperbolic 3-manifolds. The set consisting of the commensurability classes of
hyperbolic 3-manifolds admits the quotient topology induced by the geometric
topology. We show that this quotient space satisfies some separation axioms.
Roughly speaking, this means that commensurablity classes are sparsely
distributed in the space consisting of the hyperbolic 3-manifolds.
