We report the first kinetic measurement of cross-nucleation between polymorphs, a newly discovered phenomenon important to the theory and control of crystallization. D-Mannitol crystallized from its melt first as the least stable delta polymorph and then as the second least stable alpha polymorph, with alpha nucleating on delta. The kinetics of cross-nucleation was determined from the frequencies of alpha nuclei appearing on delta spherulites, the distances between alpha and delta nuclei, and the growth rate of the delta spherulite. The presence of poly(vinylpyrrolidone), a noncrystallizing, melt-miscible additive, increased the rate of cross-nucleation.