The spectral character of the stimulated emission from ruby has been investigated; we found that the zero-fieldsplitting(0.38 cm−1)of the4A2ground level and the cavity optical length and its reflectivity determinethe spectral character of this emission. At 300°K the system is induced into oscillation in a number of cavitymodes separated by&Dgr;v¯ = (2l&egr;12)−1, wherelis the length of the resonator and&egr;12is the index of refraction forthe ordinary ray. AtT≅80°KtheE¯(2E) → ±32(A2)and theE¯(2E) → ±12(4A2)transitions were identifiedin the stimulated emission spectrum. Each can oscillate in an axial cavity mode (or modes) plus an off-axismode at the center of the transition. At 4.2°K the system oscillates in off-axis modes, unless the length of thecavitylis such that it has axial resonances coincident with the transition frequencies. An analysis of thetemperature dependence of the oscillation threshold for the separate transitions is presented and is comparedwith the observed behavior.