The ultrafast photochemical ring opening reaction of 1,3-cyclohexadiene is investigated theoretically. To allow for a quantum mechanical treatment of this high-dimensional problem we introduce reactive coordinates which contain the modes active on the femtosecond time scale. A reduced Hamiltonian is derived in the corresponding subspace and two-dimensional potential energy surfaces are interpolated using ab initio data points. The quantum dynamics in the S-1 state toward two conical intersections with the ground state is monitored. We find a bifurcation of the wave packet already in the excited state so that both conical intersections are reached but by different parts of the wave packet. By manipulating the initial conditions the branching ratio can be influenced. (C) 2000 American Institute of Physics. [S0021-9606(00)00511-0].