  We present results from a dynamical study of the high redshift, massive,
X-ray luminous galaxy cluster MS1054--03. We significantly increase the number
of confirmed cluster members by adding 20 to an existing set of twelve; using
the confirmed members, we estimate MS1054--03's redshift, velocity dispersion,
and mass. We find that z=0.8329 +/- 0.0017, sigma = 1170 +/- 150 km/s, and the
central mass is approximately 1.9 +/- 0.5 x 10^{15} h^{-1} M_{odot} (within R=1
h^{-1} Mpc; H_0 =100h km s^{-1} Mpc^{-1}, q_0=0.5). MS1054--03 is one of a
handful of high redshift (z>0.5) clusters known that also has X-ray and
weak-lensing observations (Donahue et al. 1998; Luppino & Kaiser 1997); we find
our dynamical mass agrees with mass estimates from both studies. The
confirmation of MS1054--03 as a massive cluster at z~0.8 is consistent with an
open (Omega_M~0.3) or flat, Lambda-dominated (Omega_M+Omega_{Lambda}=1)
universe. In addition, we compare MS1054--03's velocity dispersion and X-ray
temperature to a sample of low and intermediate redshift galaxy clusters to
test for evolution in the sigma - T_x relation; we find no evidence for
evolution in this relation to z~0.8.
