A massive cluster of galaxies at z=0.996

We report the identification of a cluster of galaxies around the high-redshift radio galaxy 3CR 184 at z = 0.996. The identification is supported by an excess of galaxies observed in projection in I-band images (both in ground-based and Hubble Space Telescope [HST] data), a peak in the redshift distribution comprising 11 galaxies (out of 56 with measured redshifts) in a similar to 2000 km s(-1) velocity interval, and the observation on HST WFPC2 frames of a gravitational are seen projected at 42 h(50)(-1) kpc away from the central radio galaxy. We thus have strong evidence for the presence of a massive cluster at z similar or equal to 1. The mass contained within the are radius is in the range (1.20-2.78) X 10(13) h(50)(-1) M. for z(arc) between 1.5 and 3; the corresponding mass-to-light ratio varies from 56 h(50) to 140 h(50). The velocity dispersion deduced from the galaxy cluster redshifts is 634(-102)(+206) km s(-1), leading to a virial mass M = 6.16(-2.40)(+3.94) X 10(14) h(50)(-1)M. and a mass-to-light ratio of 200 h(50) < (M/L-B)(400) h(50)(-1) < 500 h(50) within a radius of 400 h(50)(-1) kpc.