The rate-controlling species for creep of fine-grained doped lanthanum gallate, La0.8Sr0.2Ga0.85Mg0.15O2.825 (LSGM), was determined by comparing: (i) the activation energy for creep with activation energies determined by ionic conductivity, oxygen tracer diffusion and theoretical predictions; (ii) the magnitude of the diffusion coefficient determined from creep data with the self-diffusion coefficients; and (iii) the dependence of the stress on oxygen partial pressure with point defect models. The results suggest that creep of LSGM is controlled by cation vacancies (La or Ga). Based on creep data for other perovskites and theoretical predictions it is likely that the defect which controls creep of LSGM is Ga vacancies.