The effect of molecular weight on the isothermal crystallization and complex melting behavior of polymorphic nylon-6 has been studied within the context of the steplike crystallization and melting mechanism. Quiescent isothermal crystallization, at relatively low crystallization temperatures, of low molecular weight nylon-6 samples, although poorly defined, predominantly developed the alpha crystalline form. Linear heating of these samples gave place to a single melting endotherm. An increase in crystallization temperature developed a better-defined a form with the single melting behavior continuing up to temperatures where a small endotherm appeared immediately after the crystallization temperature. A sequential increase of crystallization temperature (paralleled by increased definition of the alpha crystalline form) rendered triple melting behavior. This latter phenomenon was followed by the return of double melting at the expense of the third melting endotherm. after isothermal crystallization at higher temperatures. High molecular weight samples predominantly developed the gamma form at low isothermal crystallization temperatures, the melting behavior giving rise to the formation of a new melting endotherm with melting point slightly lower than the high endotherm previously observed with the low molecular weight sample. In addition, there was also the formation of a lower amount of a third melting endotherm. On the basis of these results, an explanation is given for each melting endotherm. It was also determined that, on heating, nylon-6 can recrystallize from the less stable gamma form to the more stable a form within a process parallel to melting. From a comparison of the crystallization results for different molecular weights, under the stable a crystal habit, it was determined that the basic concepts of the steplike crystallization and melting mechanism can also be applied to the complex melting behavior of the polymorphic nylon-6.