The mechanism of the photochemical 1,3-cycloaddition of ethene to benzene was investigated by the semiempirical MO method SINDO1. The relevant first excited singlet potential energy hypersurface was studied, and intermediates and transition structures were optimized with limited configuration interaction (CI). Two different reaction pathways proposed by experimentalists were compared. From the calculated barriers, a mechanism which involves a prefulvene structure is favored, whereas a simultaneous 1,3-cycloaddition via an exciplex mechanism seems most unlikely.