  We report the detection of atomic chlorine emissions in the atmosphere of Io
using Hubble Space Telescope observations with the Goddard High Resolution
Spectrograph (GHRS). The Cl I 1349 A dipole allowed and Cl I] 1386 A forbidden
transition multiplets are detected at a signal to noise ratio (SNR) of 6 and
10, respectively, in a combined GHRS spectrum acquired from 1994 through 1996.
Oxygen and sulfur emissions are simultaneously detected with the chlorine which
allows for self-consistent abundance ratios of chlorine to these other
atmospheric species. The disk averaged ratios are: Cl/O = 0.017 +/- 0.008, Cl/S
= 0.10 +/- 0.05, and S/O = 0.18 +/- 0.08. We also derive a geometric albedo of
1.0 +/- 0.4 % for Io at 1335 A assuming an SO2 atmospheric column density of
1x10^{16} cm^{-2}.
