A Search for clusters at high redshift I .

Radio, optical and X-ray observations of the powerful radio galaxy PKS 1138–262 at z = 2.156 have suggested that this galaxy is a massive galaxy in the center of a forming cluster. We have imaged 1138–262 and the surrounding 38 square arcminute field with the Very Large Telescopea in a broad band and a narrow band encompassing the redshifted Lyα emission. We detect 50 objects with rest equivalent width larger than 20 Å and a luminous, highly extended Lyα halo around 1138– 262. If the radio galaxy is at the center of a forming cluster, as observations at other wavelengths suggest, these objects are candidate Lyα emitting cluster galaxies.