The molecular rotational symmetry number, [sgrave], is defined and evaluated for a wide range of rigid organic compounds. Molecular symmetry is an important determinant of a variety of properties of crystalline substances. It is shown that the entropy of melting, ΔSm, is related to symmetry by ΔSm= 13.5–RIn[sgrave]. Since molecular symmetry plays an important role in the determination of the entropy of melting it is also important in determining vapor pressure and ideal solubility, both of which depend upon entropy of melting.