We compare the highly clustered populations of very high redshift galaxies with protoclusters identified numerically in a standard ΛCDM universe (Ω0 = 0.3, λ0 = 0.7) simulation. We evolve 2563 dark matter particles in a comoving box of side 150 h-1 Mpc. By the present day there are 63 cluster-sized objects of mass in excess of 1014 h-1 M☉ in this box. We trace these clusters back to higher redshift, finding that their progenitors at z = 4-5 are extended regions of typically 20-40 Mpc (comoving) in size, with dark halos of mass in excess of 1012 h-1 M☉ and are overdense by typically 1.3-13 times the cosmological mean density. Comparison with observations of Lyα-emitting galaxies at z = 4.86 and 4.1 indicates that the observed excess clustering is consistent with that expected for a protocluster region if Lyα emitters typically correspond to massive dark halos of more than 1012 h-1 M☉. We give a brief discussion of the relation between the high-redshift concentration of massive dark halos and present-day rich clusters of galaxies.