  We report the discovery of a highly significant concentration of galaxies at
a redshift of <z>=3.090. The structure is evident in a redshift histogram of
photometrically selected ``Lyman break'' objects in a 9' by 18' field in which
we have obtained 78 spectroscopic redshifts in the range 2.0 < z <3.4. The
dimensions of the structure projected on the plane of the sky are at least
11'by 8', or 14h_{70}^{-1} by 10h_{70}^{-1} Mpc (comoving; \Omega_M=1). The
concentration contains 15 galaxies and one faint (R=21.7) QSO. We consider the
structure in the context of a number of cosmological models and argue that
Lyman-break galaxies must be very biased tracers of mass, with an effective
bias on mass scale M~10^{15}M_{\sun} ranging from b~2 for \Omega_M=0.2 to b >~6
for \Omega_M=1. In a Cold Dark Matter scenario the large bias values suggest
that individual Lyman-break galaxies are associated with dark halos of mass
M~10^{12} M_{\sun}, reinforcing the interpretation of these objects as the
progenitors of massive galaxies at the present epoch. Preliminary results of
spectroscopy in additional fields suggest that such large structures are common
at z~3, with about one similar structure per survey field. The implied space
density is consistent with the possibility that we are observing moderately
rich clusters of galaxies in their early non-linear evolution. Finally, the
spectrum of one of the QSOs discovered in our survey (z_{em} = 3.356) exhibits
metal line absorption systems within the 3 redshift bins having the largest
number of galaxies in field, z = 2.93, 3.09, and 3.28. These results are the
first from an ongoing ``targeted'' redshift survey designed to explore the
nature and distribution of star-forming galaxies in the redshift range 2.7 <~ z
<~ 3.4.
