Nucleating agents increase the impact, strength, tensile strength, and tensile elasticity modulus of semicrystalline polymers. Nucleating agents also decrease product cycle times, resulting in a cost savings per product unit. We have synthesized and tested 15 compounds as nucleators for polypropylene. Of these, trinaphthylidene sorbitol, tri(4-methyl-1-naphthylidene)sorbitol, tri-(4-methoxy-1-naphthylidene)sorbitol, and dibenzylidene xylitol are efficient nucleators of polypropylene. Trinaphthylidene sorbitol (tns) has two major diastereomers : The "S" diastereomer yields a faster crystallization rate for polypropylene than does the commercial nucleator dibenzylidene sorbitol (Millad 3905). Crystallization rates are 208 and 88, respectively (t-1/.05 min 1 X 1000), The "R" diastereomer, however, is a poor nucleator and interferes with the nucleating activity of the "S" diastereomer. A 52/48 mixture of diastereomers does not nucleate polypropylene, even at twice the concentration. This is the first, time that the importance of stereochemistry has been demonstrated in the nucleating action.