A Vickers microindentation setup has been developed for in situ X-ray microdiffraction with synchrotron radiation at the ESRF microfocus beamline (ID13). Ex situ experiments performed on a Nylon 66 fiber show changes in chain orientation in the indented zone, but the crystallinity remains unchanged. Similar experiments for an UHMW-PE fiber show a breakup of the orthorhombic phase into domains due to plastic deformation and a partial transformation of the orthorhombic phase into the monoclinic phase. In situ experiments during indentation of an UHMW-PE fiber show domain doubling during indentation and single domain recovery after the tip has been retracted.