  We present an analysis of single-site time-series photometry of the pulsating
pre-white dwarf PG 0122+200. We show the pulsations are consistent with a
pattern of modes equally spaced in period; both the observed period range and
spacing confirm that PG 0122 is a g-mode pulsator. PG 0122 shows a pattern
similar to that seen in multi-site observations of PG 2131+066 and PG 1159-035.
The measured period spacing, combined with the spectroscopic temperature,
constrain the stellar mass much more precisely than the published measurement
of its surface gravity. Based on stellar models, the mass of PG 0122 falls in
the range 0.66-0.72 Msun. Fine structure in the power spectrum indicates that
PG 0122 rotates once every 1.6 days. Future multi--site observation (e.g.,
using the Whole Earth Telescope) should increase the precision of these results
and reveal detailed information on the internal structure of this variable
pre-white dwarf star.
