  We measure differences in the emission line flux from the circumgalactic
medium, CGM, of galaxies in different environments. Such differences could be a
critical clue in explaining a range of galaxy properties that depend on
environment. Using large samples of stacked archival spectra from the Sloan
Digital Sky Survey, we find that the H$\alpha$ + [N {\small II}] emission line
flux from the CGM within 50 kpc of $\sim$ L$^*$ galaxies is lower both for
galaxies that lie within a projected distance of $\sim$ 500 kpc from a massive
($M_*>10^{11} M_\odot$) galaxy and for galaxies in richer/denser environments.
The environmental differences are statistically significant even after we
control for galaxy mass and morphology. We interpret these observations as a
direct signature of environmentally-caused strangulation. We present a simple,
heuristic model for the effect of a massive parent galaxy. In this model, the
CGM cool gas fraction within 50 kpc is significantly decreased for galaxies
that lie within 700 kpc of a massive galaxy, with about 80\% of the cool gas
removed even when the galaxy is at a distance of 500 kpc from its massive
parent. However, we discuss alternative physical causes for the observed
behavior and discuss ways forward in addressing open questions.
