Evidence of the close relation between the AlB/sub 2/ structure-type to the Laves phases of the actinide elements is presented. It is shown that ThAl/ sub 2/ forms with ThMn/sub 2/ a true ternary Laves phase with the MgCu/sub 2/ structure type. The extent of the composition range of stability at 25 deg C is illustrated. Little terminal solid solubility exists in the pseudo-binary system contrast to the ThAl/sub 2/UAl/sub 2/ system. An electron transfer mechanism, which effectively increases the size of Al and decreases the size of Th in ThAl/ sub 2/, is suggested for the formation of the cubic Laves structure. (P.C.H.)