  We present a new study of archival ALMA observations of the CO(2-1) line
emission of the host galaxy of quasar RX J1131 at redshift $z$=0.654, lensed by
a foreground galaxy. A simple lens model is shown to well reproduce the optical
images obtained by the Hubble Space Telescope. Clear evidence for rotation of
the gas contained in the galaxy is obtained and a simple rotating disc model is
shown to give an excellent overall description of the morpho-kinematics of the
source. The possible presence of a companion galaxy suggested by some previous
authors is not confirmed. Detailed comparison between model and observations
gives evidence for a more complex dynamics than implied by the model. Doppler
velocity dispersion within the beam size in the image plane is found to account
for the observed line width.
