A direct trajectory method was employed to investigate the intramolecular dynamics and unimolecular decay of the trimethylene biradical over a range of energies. This method proved to be computationally viable when the internuclear forces were determined by semiempirical molecular orbital theory. The trimethylene decay is double exponential at low energies, but becomes single exponential with a statistical rate constant as the energy is increased. The non-statistical behavior at low energies arises from incomplete intramolecular vibrational energy redistribution (IVR). The simulated results are in good agreement with the available experimental data.