Abstract : We studied the self adhesion of semicrystalline polymers with variable degrees of crystallinity by performing fracture studies of interfaces formed between their glass transition temperature, Tg, and their melting temperature, Tm, in order to elucidate the role of crystallinity. We used a series of copolymers of polybutylene(terephthalate-co-isophthalate). For pre-crystallized samples, when the temperature of contact (Tcontact) was decreased below Tm, the fracture toughness, Gc decreased with Tcontact to vanish at a temperature corresponding to the onset of the fusion process. For quenched samples obtained in the amorphous state and put in contact before rising the temperature to Tcontact, Gc was very low for temperatures of contact well above Tg. The very thin crystalline layer observed in TEM was held responsible of this weak adhesion. The essential role played by surfaces and interfaces in nucleating crystallinity was shown to be an important factor affecting self-adhesion below Tm.