Loricariidae is a very diverse family of catfishes found primarily in the Amazon River basin. These catfishes have a unique characteristic feature of having fenestrae (holes) in the skull region (compound pterotic bone) adjacent to their bi-lobed swim bladder. Since the swim bladders and the compound pterotic may act as an external ear for hearing in this taxon, I hypothesized that these swim bladders structures have an acoustical functional in the loricariid Otocinclus affinis. In order to understand the development of these structures in O. affinis, I first monitored the ontogeny of the compound pterotic bone by clearing and staining of fish ranging total length from 0.75 to 3.5 cm. the swim bladders and fenestrated compound pterotic bone were developed at early larval stages (by 5.25 mm TL). Second I examined the role of swim bladders in hearing in adult O. affinis, by testing the hearing sensitivity before and after swim bladder deflation. Hearing thresholds were determined electrophysiologically by recording auditory evoked potentials (AEP). Swim bladder deflation increased hearing thresholds by 19 – 23 decibels (dB). I then tested the role of the compound pterotic bone fenestrae in hearing by covering them with a tissue adhesive (n-butyl cyanoacrylate) and recording the hearing thresholds. Covering fenestrae increased hearing thresholds by 4 – 11 dB. Future experiments are required to more precisely determine the acoustical role of these peripheral auditory structures in O. affinis.