  The cluster of galaxies A754 undergoes a merger of several large structural
units. X-ray observations show a nonequilibrium state of the central part of
the cluster, in which a cloud of cold plasma 500 kpc in size was identified
amid the hotter cluster gas. The X-ray image of A754 exhibits a brightness
discontinuity, which can be interpreted as a shock wave in front of a moving
cloud of dense gas. The shock parameters are determined from the jump in
intergalactic gas density using the ROSAT image. The estimated Mach number is
M=1.71^{+0.45}_{-0.24} at a 68% confidence level.
