Microstructural changes in epitaxial SrRuO3 thin films grown on SrTiO3(001) during the early stages of growth by pulsed laser deposition were studied using synchrotron x ray scattering. We find that at a very early stage of growth (≤110 Å), the film is composed of a single (110) domain only, in sharp contrast to double (110) domains reported previously for thick (≥1000 Å) films. As the film grows further (≫110 Å), another, 90° rotated, (110) domain starts to form, coexisting with the original (110) domain. This domain evolution is attributed to strain relaxation, eventually resulting in significant atomic ordering in the in-plane direction. © 2001 American Institute of Physics.