The isotopic composition of argon entrapped in a 6.3 carat, Type I, Arkansas, U. S. A. diamond (emplacement data 106¿3 m.y. ago) has been determined by mass spectrometric techniques. This was done by crushing the stone in a diamond crusher attached to the high vacuum inlet system of a high-sensitivity research mass spectrometer. The results show an 40Ar/36Ar value of 189 compared to a ratio of 294 for atmospheric argon, and an 38Ar/36Ar value of 0.18 almost identical to that for atmospheric argon (0.19). The isotopic data have been applied to a model of the outer Earth to test the crystallization age of the diamond. The results may be interpreted in terms of diamond crystallization about 3.1 billion years ago. The existence of primordial 36Ar and 38Ar (non-radiogenic isotopes) shows that the Earth had not completely degassed at the time of diamond crystallization. In fact, if the diamond is assumed to be a representative sample of the upper mantle with respect to fluid content, the mass of primordial 36Ar in this region of the Earth is calculated to be 1¿1010 tons at the time of diamond crystallization, or 2.5¿10-5 ppm by weight.