The dynamic of molecular mobilities occurring above and below the glass transition of a polyamide called MXD6 have been investigated on samples with different degrees of crystallinity. The fragility index (m) has been evaluated for a fully amorphous material and it was found that m = 180 classifies this material as fragile in regard to the strong-fragile glass forming liquid concept. For semi-crystalline samples, the results show two relaxation processes at the glass transition. One can be attributed to the molecular mobility existing in the free amorphous domain, and the second peak to a molecular mobility existing in a rigid amorphous domain as proposed by the '3-phase model'. The existence of these two phenomena does not allow a correct determination of the fragility index for such semi-crystalline materials by means of calorimetric investigation.