  We report high precision, high cadence photometric measurements of the star
HD 209458, which is known from radial velocity measurements to have a planetary
mass companion in a close orbit. We detect two separate transit events at times
that are consistent with the radial velocity measurements. In both cases, the
detailed shape of the transit curve due to both the limb darkening of the star
and the finite size of the planet is clearly evident. Assuming stellar
parameters of 1.1 R_Sun and 1.1 M_Sun, we find that the data are best
interpreted as a gas giant with a radius of 1.27 +/- 0.02 R_Jup in an orbit
with an inclination of 87.1 +/- 0.2 degrees. We present values for the
planetary surface gravity, escape velocity, and average density, and discuss
the numerous observations that are warranted now that a planet is known to
transit the disk of its parent star.
