  We present the results of ROSAT High Resolution Imager (HRI) observations and
the survey data of the radio-loud Narrow-Line Seyfert 1 galaxy (NLS1) PKS
0558--504. We find strong and persistent X-ray variability on both short and
medium time-scales. The most extreme amplitude variations require a radiative
efficiency exceeding the theoretical maximum for a Schwarzschild black hole,
suggesting the presence of a rotating black hole or the influence of
relativistic beaming effects. The spatial analysis rules out the possibility
that the high luminosity and the strong variability are related to a nearby
source.
