CASSCF(4,4), MR-CI and CASSCF(4,4)+MP2 calculations using the basis sets 3-21G* and 6-31G* have been performed in a study of the reaction between ethylene and silene. A cyclic transition state for the interaction leading on one side to a planar silacyclobutane transition state (TS), which falls to ground-state puckered silacyclobutane, and on the other side to a trans diradical having strict C, symmetry has been found in this multistep process, At the highest calculational level the energy of this TS is 26.9 kcal/mol above a supermolecule consisting of noninteracting ethylene and silene molecules. The trans diradical and its gauche form have about the same energy, viz., 15,8 and 16.7 kcal/mol, respectively, above the same reference. The TS for fragmentation of the trans Corm to ethylene and silene is 16.0 kcal/mol above this reference and is 2.4 kcal/mol lower than the barrier to interconversion between the trans and the gauche forms, Size-consistency corrections to the MR-CI results are of great importance for semiquantitative estimates of energies, A symmetry analysis oi the potential surface shows our proposed reaction paths are consistent.