We report on the fabrication of solution-processed organic phototransistors (OPTs) based on perylenebis(dicarboximide)s (PDIs). We found that the responsivity to the photoillumination depends on the transistor's channel length and that it can be tuned by varying the device geometry. The analysis of different morphologies of the active semiconducting layer revealed. that single PDI fibers exhibit the higher photoresponse when compared to more poorly organized films. The highest responsivity value of 4.08 +/- 1.65 x 10(5) A/W was achieved on a multifiber-based OPT. These findings represent a step forward toward the use of organic based phototransistors as photosensors.