  Recent work has shown that the star formation-density relation -- in which
galaxies with low star formation rates are preferentially found in dense
environments -- is still in place at z~1, but the situation becomes less clear
at higher redshifts. We use mass-selected samples drawn from the UKIDSS
Ultra-Deep Survey to show that galaxies with quenched star formation tend to
reside in dense environments out to at least z~1.8. Over most of this redshift
range we are able to demonstrate that this star formation-density relation
holds even at fixed stellar mass. The environmental quenching of star formation
appears to operate with similar efficiency on all galaxies regardless of
stellar mass. Nevertheless, the environment plays a greater role in the
build-up of the red sequence at lower masses, whereas other quenching processes
dominate at higher masses. In addition to a statistical analysis of
environmental densities, we investigate a cluster at z=1.6, and show that the
central region has an elevated fraction of quiescent objects relative to the
field. Although the uncertainties are large, the environmental quenching
efficiency in this cluster is consistent with that of galaxy groups and
clusters at z~0. In this work we rely on photometric redshifts, and describe
some of the pitfalls that large redshift errors can present.
