The conformation of 1,3-benzodioxole has been examined using ab initio calculation and natural bond orbital (NBO) analysis in order to find the origin of its unusual nonplanarity. Geometry optimizations for the planar (C-2v) and flap-puckered (C-s) conformers of 1,3-benzodioxole have been performed at the HF, B3LYP, and MP2 levels, and the results indicate that the flap-puckerd conformer is more stable than the planar conformer. High-level electron correlation treatments with extended-basis sets have also been performed to provide a reliable prediction of the puckering barrier for 1,3-benzodioxole. The calculated puckering barrier appears to be in reasonable agreement with the experiment, but the divergent behavior of the Moller-Plesset series suggests that it is impossible with conventional basis sets smaller than 400 functions to converge the barrier height. NBO analysis of tile Hartree-Fock wave functions shows that the conformational preference of the C-s conformer over the C-2v is the result of a wide variety of hyperconjugative orbital interactions, but the interaction between the oxygen lone pair (n(p)) and the o*(CO) orbital, which is closely associated with the anomeric effect, is the most important factor favoring the nonplanar conformation. However, 1,3-benzodioxole has a lower puckering barrier to planarity than 1,3-dioxole due to the suppression of the anomeric effect by the benzene ring.