The crystallization of nylon-6 from the melt was monitored in situ by X-ray diffraction. The nylon-6 was found to crystallize into a high-temperature alpha'-phase as indicated by the two-peak nature of the diffractogram. On cooling from the crystallization temperature to room temperature, nalon-6 retained the two-peak nature. However, data analysis indicates a change from high temperature (HT) alpha'-phase to low-temperature alpha -phase at similar to 180 degreesC. On heating, the a-phase transformed into the alpha'-phase at about 190 degreesC and melted in the alpha'-phase. The transition took place over a temperature range where both phases coexisted. However, samples crystallized from the melt at temperatures 140 and 180 degreesC showed the alpha -phase at room temperature, but on heating the alpha -phase first transformed into a pseudohexagonal phase and before melting the pseudohexagonal phase further transformed into the alpha'-phase. The alpha -phase was transformed into the gamma -phase, by potassium iodide-iodine treatment, and the behavior of the gamma -phase with temperature had been studied for the first time. The gamma -phase was very stable and did not show any crystalline transition below the melting point.