A theory describing the optoacoustic signal is presented; dependence on both cell and gas parameters are given and the advantage of operating at a resonant frequency is discussed. Three elliptical cells with major axes 5.8, 12.7, and 15.2 cm (corresponding minor axes 5.5, 11.0; and 7.6 cm) were utilized as resonant optoacoustic cells. Longitudinal standing waves analagous to radial resonances for a cylindrical cell were driven by passing the beam from a CO2 laser along one focus of the ellipse. Results are compared with measurements on a Helmholtz cell for which the minimum detectable absorption coefficient was about 2 x 10 to the -7th power cm. Windowless operation was attempted, and the results indicate that further improvement is possible.