Both self-consistent field and configuration interaction calculations have been performed with several different basis sets to determine the low-lying excited states of bicyclobutane (C4H6). The only low-lying excited states found to exist below 7.0 eV contained substantial Rydberg character. Oscillator strengths calculated for those states with vertical excitation energies close to 6.7 eV indicate that three states are capable of forming C4H6 isomers when bicyclobutane is photolyzed at 6.7 eV. A detailed discussion of probable pathways for the photochemical decomposition of bicyclobutane to its isomeric partners, butadiene and cyclobutene, is given. © 1991 American Chemical Society.