  We identify the extended Einstein IPC X-ray source, 1E0657-56, with a
previously unknown cluster of galaxies at a redshift of $z=0.296$. Optical CCD
images show the presence of a gravitational arc in this cluster and galaxy
spectra yield a cluster velocity dispersion of $1213^{+352}_{-191}$ km
s$^{-1}$. X-ray data obtained with the ROSAT HRI and ASCA indicate that
1E0657-56 is a highly luminous cluster in which a merger of subclusters may be
occurring. The temperature of the hot gas in 1E0657-56 is $\rm{kT}=17.4 \pm 2.5
keV$, which makes it an unusually hot cluster, with important cosmological
implications.
