  We present the first ground-based detection of sodium absorption in the
transmission spectrum of an extrasolar planet. Absorption due to the atmosphere
of the extrasolar planet HD189733b is detected in both lines of the NaI
doublet. High spectral resolution observations were taken of eleven transits
with the High Resolution Spectrograph (HRS) on the 9.2 meter Hobby-Eberly
Telescope (HET). The NaI absorption in the transmission spectrum due to
HD189733b is (-67.2 +/- 20.7) x 10^-5 deeper in the ``narrow'' spectral band
that encompasses both lines relative to adjacent bands. The 1-sigma error
includes both random and systematic errors, and the detection is >3-sigma. This
amount of relative absorption in NaI for HD189733b is ~3x larger than detected
for HD209458b by Charbonneau et al. (2002), and indicates these two
hot-Jupiters may have significantly different atmospheric properties.
