  We present RIz photometry of four consecutive transits of the newly
discovered exoplanet XO-1b. We improve upon the estimates of the transit
parameters, finding the planetary radius to be R_P = 1.184 +0.028/-0.018
R_Jupiter and the stellar radius to be R_S = 0.928 +0.018/-0.013 R_Sun,
assuming a stellar mass of M_S = 1.00 +/- 0.03 M_Sun. The uncertainties in the
planetary and stellar radii are dominated by the uncertainty in the stellar
mass. These uncertainties increase by a factor of 2-3 if a more conservative
uncertainty of 0.10 M_Sun is assumed for the stellar mass. Our estimate of the
planetary radius is smaller than that reported by McCullough et al. (2006) and
yields a mean density that is comparable to that of TrES-1 and HD 189733b. The
timings of the transits have an accuracy ranging from 0.2 to 2.5 minutes, and
are marginally consistent with a uniform period.
