Singlet exciton fission (SF) in organic chromophore assemblies results in the conversion of one singlet exciton (S-1) into two triplet excitons (T-1), provided that the overall process is exoergic, i.e., E(S-1) > 2E(T-1). We report on SF in thin polycrystalline films of two terrylene-3,4:11,12-bis(dicarboximide) (TDI) derivatives 1 and 2, which crystallize into two distinct pi-stacked structures. Femtosecond transient absorption spectroscopy (fsTA) reveals a charge-transfer state preceding a 190% T-1 yield in films of 1, where the pi-stacked TDI molecules are rotated by 23 degrees along an axis perpendicular to their pi systems. In contrast, when the TDI molecules are slip-stacked along their N-N axes in films of 2, fsTA shows excimer formation, followed by a 50% T-1 yield.