The relaxation of heavily carbon doped GaAs, grown on (001) GaAs substrates was investigated using transmission electron microscopy and x-ray diffraction. Misfit dislocation introduction occurred only significantly above Matthews and Blakeslee’s critical thickness, with the onset of relaxation being delayed as the C concentration was increased. The resultant strain relaxation was highly anisotropic with the introduction of As-cored misfit dislocations being totally suppressed at the highest C levels, resulting in cracking of the epilayer. Locking of As-cored partial dislocations and a reduction in misfit dislocation velocity is proposed as the cause of the anisotropy and lack of relaxation. ©1997 American Institute of Physics.