The T-1 <-- S-0 electronic band spectrum in acetaldehyde was simulated from RHF and UHF/MP2/6-311G(d,p) ab initio calculations for the two electronic states. The torsion-wagging energy levels were evaluated by the variational method using free rotor basis functions expressed as symmetrized double-Fourier expansions. A comparison of the calculated band spectrum to the long-path absorption spectrum allowed for the assignment of a number of clearly defined bands and placed the origin at 27 240 cm(-1). The calculated height of the barrier to internal rotation for the lower S-0 state of 415 cm(-1) yielded levels in excellent agreement with the torsional levels derived from far infrared measurements. The calculated T-1 excited-state barrier heights for the pure torsion and wagging motions were 647 and 968 cm(-1). The corresponding values in the S-1 state are 709 and 776 cm(-1).