Abstract: A modular drilling system including a plurality of different size drill bits which can be interchangeably mounted on a common tapered male connecting portion of a drill rod assembly. The tapered male connecting portion may be formed directly onto a tapered drill rod of the assembly or may be formed on an adapter which is threaded onto a threaded drill rod of the assembly. Each bit includes a female connecting portion having a generally cylindrical tapered surface mating with a corresponding tapered surface of the male connecting portion and at least three planar sections adapted to provide positive engagement with corresponding planar sections of the male connecting portion.