The potential energy surface for the simultaneous methyl and asymmetric aldehydic torsions in propanal has been determined at the restricted Hartree-Fock/second-order Moller-Plesset level using the 6-311(3df,p) basis set. The fit of the energy values to a symmetry adapted functional form was carried out in two ways : using all the 56 energy values obtained in the optimization procedure, or dropping out the energy values which do not respect the C-3 dynamical symmetry of the methyl group, i.e., using only 28 values. With these potentials, as well as with the kinetic parameters and the electric dipole moment variations, the far infrared frequencies and intensities for the methyl and aldehyde torsions were determined theoretically. It is found that the two-dimensional calculation for the cis conformer satisfactorily reproduces not only the methyl torsion and asymmetric aldehyde torsion spectrum, but furnishes also methyl torsionally excited progressions for the aldehyde torsion modes. Most of these transitions occur in the region of 220-100 cm(-1) and are responsible for the complexity of the spectrum. From these theoretical results, a new assignment is proposed. The fit of the energy values which respect to the C-3 dynamical symmetry seems to produce better results and also permits one to save computational time.