The influence of the degree of stereoregularity on the overall rate of crystallization and on the melting behaviour of fractions of isotactic polypropylene has been investigated. Fractions with different stereoregularity were obtained by extraction methods. The stereochemical pentad population was determined by ¹³C n.m.r. analysis. The results show that at constant T_c the overall rate constant of crystallization decreases with the increase of configurational chain defects whereas at a given ΔT the contrary is observed. An evident phenomenon of secondary crystallization is observed only in fractions of IPP with a higher concentration of defects. Such observation may explain the double peaked endotherm of fusion of more defective IPP fractions. The free energy of formation of a nucleus of critical dimensions has been calculated for each fraction and its value correlated with the degree of stereoregularity.