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I present optical and near--infrared observations of two radio galaxies at $z=1.2$. The infrared images were obtained with the new large--format (NICMOS~III) infrared imaging camera (IRIM) at KPNO, and reveal the presence of rich galaxy clusters around the radio sources. The clusters are essentially invisible in deep optical images, as they appear to be dominated by a population of galaxies with extremely red colors ($R-K > 5.5$) similar to those expected for relatively unevolved early--type galaxies at $z > 1$. Deep images taken with the same instrument of other (non--radio galaxy) fields confirm that galaxies with such red colors are uncommon among random samples of faint field galaxies, and that these apparent clusters around the radio galaxies represent a very strong statistical excess.
The implications of these observations for high redshift cluster surveys, radio galaxy and quasar environments, and color evolution of galaxy cluster populations are briefly considered.
