Differential and integral electron excitation cross sections for the O2 a1Δg←X3&Sgr;g- transition at 0.97 eV have been measured as a function of incident electron energy from 2.6 to 28.6 eV. Differential cross sections (DCS) for the a1Δg state were measured relative to elastic scattering as a function of scattering angle at each incident energy for the angular range 20¿--134¿. At incident energies between 7 and 18 eV, it was necessary to use a synthetic spectrum program to separate the a1Δg←X3&Sgr;g- band intensity from intense vibronic excitation of the O2 ground state. The relative DCS values were converted to absolute values using the elastic scattering cross-section values of Trajmar et al. (1971) up to 90¿ and revised values at higher angles. The DCS results, which show the typical weak dependance on scattering angle of an optically forbidden transition, are in satisfactory agreement with previous measurements. The DCS were integrated to give integral cross sections at each incident energy. The results are in excellent agreement with previous measurements and help to establish the a1Δg←X3&Sgr;g- transition cross section as a valuable secondary standard for the measurement of electron excitation cross sections in unstable species such as atomic oxygen. ¿ American Geophysical Union 1992