We present z-band photometry of three consecutive transits of the exoplanet TrES-1, with an accuracy of 0.15% and a cadence of 40s. We improve on estimates of the system parameters, finding in particular that the planetary radius is 1.081+/-0.029R_Jup_ and the stellar radius is 0.011+/-0.020R_{sun}_. The uncertainties include both the statistical error and the systematic error arising from the uncertainty in the stellar mass. The transit times are determined to within about 15s and allow us to refine the estimate of the mean orbital period: P=3.0300737+/-0.0000026days. We find no evidence for starspots or other irregularities that have been previously reported.