CASSCF and CASPT2N/6-31G* calculations on silacyclopropane (1) find that the transition state for hydrogen migration in concert with ring opening, leading to ethylsilylene (2), has an energy that is significantly lower than the energies of the 1-sila- and 2-silatrimethylene diradicals (6 and 7), formed by ring opening alone. (12/12)CASPT2N calculations, corrected for differences in zero-point energies, give barriers of 24.7 and 13.2 kcal/mol, respectively, for the silacyclopropane to ethylsilylene reaction and its reverse. These values are in reasonable agreement with experimental estimates of 23-28 and 10.4 kcal/mol, respectively, for the corresponding reactions of alkylsilacyclopropanes. Diradicals 6 and 7, which are transition states, are computed to be ca. 20 kcal/mol higher in energy than the transition state leading to ethylsilylene (2). The barrier to the conversion of 2 to 1-silapropene (3) is calculated to be 30.7 kcal/mol. The strong preference for opening of silacyclopropane (1) to ethylsilylene (2), which contrasts with the opening of cyclopropane to the trimethylene diradical, is interpreted in terms of the relative Si-H and C-H BDE's in primary silyl and alkyl radicals.