We have studied the evolution of the magnetic, electronic, and structural properties of annealed epilayers of Ga _1-x Mn _x As grown by low temperature molecular beam epitaxy. Annealing at the optimal temperature of 250 ° C for less than 2 h significantly enhances the conductivity and ferromagnetism, but continuing the annealing for longer times suppresses both. These data indicate that such annealing induces the defects in Ga _1-x Mn _x As to evolve through at least two different processes, and they point to a complex interplay between the different defects and ferromagnetism in this material. © 2001 American Institute of Physics.