Singlet exciton fission is a spin-allowed process to generate two triplet excitons from a single absorbed photon. This phenomenon offers great potential in organic photo-voltaics, but the mechanism remains poorly understood. Most reports to date have addressed intermolecular fission within small-molecular crystals. However, through appropriate chemical design chromophores capable of intramolecular fission can also be produced. Here we directly observe sub-100 Is activated singlet fission in a semiconducting poly-(thienylenevinylene). We demonstrate that fission proceeds directly from the initial 1B(u) exciton, contrary to current models that involve the lower-lying 2A(g) exciton. In solution, the generated triplet pairs rapidly recombine and decay through the 2A(g) state. In films, exciton diffusion breaks this symmetry and we observe long-lived triplets which form charge-transfer states in photovoltaic blends.