The microwave spectrum of the normal isotopic species of anisole was observed in the frequency ranges 8–18 and 26.5–40 GHz. Rotational constants were obtained for the ground vibrational state and the excited states of the torsional vibration of the phenyl group. The values of Δ I(= I a + I b — I c) for the ground (Δ I o = 3.140 uÅ 2) and the first excited states (Δ I 1 = 4.222 uÅ 2) indicate that the equilibrium conformation of anisole has a planar heavy-atom skeleton. Using these Δ I values, the torsional frequency was estimated to be 83.0(3.7) cm −1. The fitted values of the r o-structural parameter are r C(1)O = 1.399 Å and ∠C(1)OC(methyl) = 113.8°. The other parameters are assumed to be the values of the fully optimized structure from the ab initio MO calculation.