The effective ground state potential energy function of the ozone molecule near the C 2v equilibrium configuration was obtained in a least-squares fit to the largest sample of experimental, high-resolution vibration-rotation data used for this purpose so far. The fitting is based on variational calculations carried out with the extended Morse Oscillator Rigid Bender Internal Dynamics model. The potential function is expanded in Morse-type functions of the stretching variables and in cosine of the bending angle. The present calculation produces results in significantly better agreement with experiment than previous determinations of the potential energy surface, and the energies predicted with the new surface are sufficiently accurate to be useful for the assignment of new high-resolution spectra. The rms (root-mean-square) deviation of the fit of rovibrational data up to J ϭ 5 is 0.02 cm Ϫ1 . For the set of all 60 band centers of the 16 O 3 molecule included in the Atlas of Ozone Line Parameters, the rms deviation is 0.025 cm Ϫ1 , and for all band centers determined so far from high-resolution spectra, including those recently observed and assigned in Reims corresponding to highly excited stretching and bending vibrations (v 1 ϩ v 2 ϩ v 3 ϭ 6), the rms deviation is 0.1 cm Ϫ1 . The "dark states" that produce resonance perturbations in the observed bands are described with experimental accuracy up to the (v 1 v 2 v 3 ) ϭ (080) state. Extrapolation tests demonstrate the predictive power of the potential function obtained: rotational extrapolation up to J ϭ 10 for the 11 lowest vibrational states results in an rms deviation of 0.06cm Ϫ1 . Also, vibrational energies measured by low-resolution Raman spectroscopy (which were not included in the input data for the fit) are calculated within the experimental accuracy (rms ϭ 1.6 cm Ϫ1 ) of the experimental values up to the dissociation limit. The statistical analysis suggests that the accuracy of the equilibrium geometry and force constants of the molecule is considerably improved relative to previous determinations. The long-range behavior of the fitted potential at the dissociation limit O 3 3 O 2 ϩ O shows very good agreement with experimental data. The new potential energy surface was used to predict the band centers of the isotopomers 17 O 3 and 18 O 3 .