X-Ray Lang topography is used to investigate the dislocation substructure developped under stress in slightly n-doped InSb samples. Results confirm the much higher velocity of α-dislocations compared to screws. On the other hand, β-dislocations are excessively little mobile, apparently at all stress and temperature. This has for consequence : 1) to affect the mobility of screw dislocations whose motion looks to be controlled by the propagation of single α-kinks. 2) to prevent the development of efficient internal sources, except near surfaces. This behaviour induces a plasticity which appears to be very specific to III-V compounds