Reaction of ethene with singlet [C-13]methylene, generated photochemically from [C-13]diazomethane at four pressures in the range 35-800 Torr, reveals rearrangement of excited [3-C-13]propene (3), the product of insertion, to [1-C-13]propene (1) and [2-C-13]propene (2). This previously unrecognized reaction is formulated as the reverse of the thermal rearrangement of cyclopropane to propene, well known in both thermal and chemically excited modes. The origin of a small excess of 1 over 2 is discussed. Mercury sensitization does not alter the observations at 820 Ton.