The presence of amplified spontaneous emission (ASE) by optical pump in polystyrene films doped with N,N′-di(10-nonadecyl)perylene-3,4:9,10-tetracarboxylic diimide (PDI-N) in a range of PDI-N concentrations between 0.25 and 5 wt. % is reported. Gain coefficients up to 10{\text{cm}}^{-1} , at a pump intensity of 74\text{kW}/{\text{cm}}^{\text{2}} , were obtained. The lowest thresholds \left(\sim 15\text{kW}/{\text{cm}}^{\text{2}}\right) and largest photostabilities measured at 50% ( \sim 50\min , i.e., 30,000 pump pulses) were obtained for concentrations up to 1 wt. %. The observation of an increase in the ASE threshold and a decrease in the photostability for larger concentrations is attributed to the presence of aggregated species.