Abstract: The endoparasitic helminths of the gastrointestinal tracts of mochokid upside-down catfish, Synodontis membranaceus (Geoffroy Saint Hilarie, 1809) caught from Jebba Lake, Nigeria with gill nets of various mesh sizes were examined for 24 months (April 2002-March 2004). Out of 1,208 fish specimens examined, 438 (36.25%) were infected with 1,321 helminth parasites, comprising two nematodes (Procamallanus laevionchus and Cucullanus species) and a cestode (Polyonchobothrium species). Parasite population data showed that nematodes had higher prevalence (27.81%) and mean intensity (03.29) than cestode (prevalence: 08.44%; mean intensity: 02.09). The prevalence of endoparasites was highest in the intestine, while the oesophagus recorded the lowest. Multiple infections and isolation of parasites from the liver and gall bladder are reported for the first time in Synodontis species. There was no significant difference (p>0.05) between the infestation of male and female specimens. However, the relationship between host size and parasite burden showed that fish below 20 cm SL were free from infection, while infection increased from 27.68% in fish between 20-27 cm SL to a significantly higher (p<0.05) value (67.61%) in fish above 27 cm SL. Infestation of endoparasites was more prevalent (p<0.05) in the dry season than in the rainy season. The implications of these results on the management and culture of Synodontis membranaceus are discussed.