We have detected the neutral atomic hydrogen (HI) emission line at a cosmol
ogically significant distance [redshift (z) = 0.18] in the rich galaxy clus
ter Abell 2218 with the Westerbork Synthesis Radio Telescope. The HI emissi
on originates in a spiral galaxy 2.0 h(65)(-1) megaparsecs from the cluster
core. No other significant detections have been made in the cluster, sugge
sting that the mechanisms that remove neutral gas from cluster galaxies are
efficient. We infer that fewer than three gas-rich galaxies were accreted
by Abell 2218 over the past 10(9) years. This low accretion rate is qualita
tively consistent with low-density cosmological models in which clusters ar
e largely assembled at z > 1.