The stability of a mixed < c + a > dislocation on the pyramidal I plane in magnesium is studied using molecular dynamics simulations. The dislocation is metastable and undergoes a thermally-activated transition to either a sessile, basal-dissociated < c + a > or a sessile basal-dissociated < c > dislocation plus an < a > dislocation. The transition is intrinsic to pure magnesium and occurs with an energy barrier of similar to 0.3 eV. The transformed structure is also consistent with experimental evidence in Ti and Zr, where pyramidal I slip is more prevalent. Enhancing the ductility of magnesium by stabilizing < c + a > slip on pyramidal I planes thus appears unlikely to be viable.