Differential scanning calorimetry was utilized for investigating the isothermal crystallization of a poly(3-hexylthiophene) (P3HT) sample over a wide range of temperatures. We found two well separated crystal formation processes : a fast process, essentially completed before the sample reaches the isothermal conditions, which produces crystals having higher transition temperatures (at around 178 degrees C), and a slow process which leads to continuous growth of less stable crystals, the transition temperatures of which are determined by the crystallization temperature, namely, about 32 degrees C above. The slow crystallization process occurs at temperatures as low as 40 degrees C. This crystallization behaviour supports the suggestion that a nematic stale exists in the melt of P3HT.