The South American gymnotiform knifefish, Brachyhypopomus gauderio is a weakly electric fish species possessing an electric organ (EO). Development of the EO at embryonic and early larval stages has not been extensively studied. B. gauderio is relatively easy to keep in the aquarium and to obtain embryos and therefore could be a good animal model for studying development of the EO. However, little is known about its embryonic development. In this study, we described fertilisation in B. gauderio until early larval stage including 32 stages (180hpf), and determined the larval hatching that was between 3-4 days. The analyses showed a large EO at the ventral side of the muscle (veo). We also discovered another small potential EO-like tissue located at the dorsal side of the muscle (deo) and that has not been previously described. The results also described the derivative electrocytes from the electroprimordium which are separated from the ventral somite muscle in the 3.5dpf during the hatching period. This structure is further separated in the following days. At that time, the first myogenic EO is developed at the border between the ventral somite and the ventral tail fin. During progression of electroprimordium development, the location of the electroprimordium moves down ventrally in the tail fin where newly formed electrocytes appear with multiple nuclei seen in a large syncytial cell. Fluorescent immunohistochemistry showed that MF20 (myosin heavy chain) antibody stained muscles at stages 60hpf, 84hpf, 5.5dpf and 7.5dpf. MF20 also weakly stained the border of electrocytes and the electroprimordium at stages 5.5dpf and 7.5dpf. PAX7 antibody was expressed in the electroprimordium at stages 5.5dpf and 7.5dpf. Moreover, in situ hybridisation staining with scn4aa suggests that cell fate specification of EO may start even earlier at embryonic stage (40hpf). We investigated signalling pathways in EO development using specific inhibitors or activator for key signalling pathways (Bmp, Nodal, Fgf, Retionic Acid and Sonic hedgehog) to the B. gauderio at early stages using in situ hybridisation with scn4aa probe. The results show that Bmp inhibitor, FGF inhibitor and RA suppressed or reduced the scn4aa expression but also reduced the tail and tail fin development. Therefore it was not obvious if these signalling pathways are directly involved in the EO development or if the signals indirectly regulate the EO development by altering cell fates of tail tissues. The SHH inhibitor, cyclopamine treatment at late blastula stage suppressed scn4aa. These results suggest that sonic hedgehog at the late blastula to early gastrula stage has a crucial role in the EO development. Edar morpholino injection suppressed the dorsal EO via deletion of ventral and dorsal fins which was confirmed by the suppression of scn4aa expression in the morphant. This could be a result of the absence of the fin which may possibly be important for activating scn4aa in the dorsal side. To study EO development in the adult fish, the caudal filament was amputated. We observed that tail regeneration occurred during the following two weeks including newly regenerated EO, suggesting that this system would be useful for studying EO development using adult fish, and for learning mechanisms of tissue regeneration in the fish. In the current study, Zn-12 antibody detected a neurogenic EO of electric fish B. gauderio that has not been discovered before. The staining signal with Zn-12 was obvious in the neurogenic EO at adult stage, however the signal at 25dpf was faint and small, and was not detected in the larvae suggesting this organ is specific to juvenile to adult stage. Furthermore, detailed description of the EO structure at early and late stages were conducted using TEM analysis, showing nuclei, ribosomes, glycogen, endoplasmic reticulum, late endosomes and large number of mitochondrion from embryonic electroprimordium to adult EO. At 8.5dpf, the electroprimordium was seen in the front line of the EO moving towards the ventral end of the fin during the EO development. TEM analyses also showed intense myelin layers formed in the neourocytes of neurogenic EO at adult stage (7mpf) considering its important in neourocytes structure. Throughout these studies, we found that the knifefish, B. gauderio embryos and adult fish with the regeneration assay are excellent models for studying development of the electric organ.