  Recent studies have found the Type II-plateau supernova (SN) 1999gi to be
highly polarized (p_max = 5.8%, where p_max is the highest degree of
polarization measured in the optical bandpass; Leonard & Filippenko 2001) and
minimally reddened (E[B-V] = 0.21 +/- 0.09 mag; Leonard et al. 2002). From
multiple lines of evidence, including the convincing fit of a ``Serkowski''
interstellar polarization (ISP) curve to the continuum polarization shape, we
conclude that the bulk of the observed polarization is likely due to dust along
the line of sight (l-o-s), and is not intrinsic to SN 1999gi. We present new
spectropolarimetric observations of four distant Galactic stars close to the
l-o-s to SN 1999gi (two are within 0.02 degrees), and find that all are null to
within 0.2%, effectively eliminating Galactic dust as the cause of the high
polarization. The high ISP coupled with the low reddening implies an
extraordinarily high polarization efficiency for the dust along this l-o-s in
NGC 3184: ISP / E(B-V) = 31^{+22}_{-9} % mag^{-1}. This is inconsistent with
the empirical Galactic limit (ISP / E[B-V] < 9% mag^{-1}), and represents the
highest polarization efficiency yet confirmed for a single sight line in either
the Milky Way or an external galaxy.
