The rotational spectrum of the O-3-CH4 complex has been measured in a molecular beam using a pulsed-nozzle Fourier-transform microwave spectrometer. An a-type pure-rotation and a c-type rotation-inversion electric-dipole spectrum is observed, complicated by the nearly free internal rotation of the CH4 top and the inversion tunneling of the O-3. The nuclear-spin statistics of the equivalent oxygen nuclei leads to only one tunneling component existing for each rotation-internal-rotation state, indicating that the transition state has a heavy-atom, C-2v-symmetry geometry. The tunneling splitting is determined to be 30 to 40 MHz, dependent on the CH4 internal-rotor state. Only two of the three methane internal-rotor states have been assigned. These two states of A and F symmetry have asymmetric-rotor energy-level structures, weakly perturbed by the ozone-inversion tunneling. The zero-point structure of the complex has a heavy-atom plane of symmetry with the two terminal O atoms equidistant above and below this plane. The angle between the line joining the center of masses of the two subunits and the O-3, C-2 axis is 118.2(5)degrees, with the central O directed away from the CH4. The shortest O-C separation is 3.57 Angstrom. The geometry of the complex suggests two outcomes for the reaction of an O atom produced by 267 nm photolysis of O-3 in the complex (assuming that the initial O-3 photodissociation dynamics are not perturbed by complexation), either nonreaction or reaction by stripping of a hydrogen atom at high impact parameters, leading to fast, highly rotationally excited, OH product. [S0021-9606(00)00430-X].