A plumb bob is disclosed that consists of a relatively low-density plastic frustoconical outer shell formed about a relatively high density metallic core. The plumb bob includes a conical support collar at its smaller end, the collar defining an axial bore for receiving either a threaded conventional plumb bob point or a conventional concrete nail. At its other end, the plumb bob defines a smooth curved surface forming a knobbed extension leading to an axial bore for fixing the supporting string. This smooth curve of the knobbed extension prevents undue wear to the supporting string when the plumb bob is in use. The construction of the plumb bob is such that the outer shell and the knobbed extension may be made of impact resistant polycarbonate plastic having a high visibility color.