Analyses of stomach contents of (1093)Heterotis niloticus(Cuvier 1829) in the River Ogbese, Owena Reservoir and Mahin Lagoon (southwestern Nigeria) established it as a benthic feeder in the river and a planktonic feeder in the reservoir and lagoon. There was a preponderance of zooplankton, accounting for > 80% occurrence and > 25% of total volume in the stomachs of specimens from all habitats; this suggests a microphagous habit. Digestive enzyme assays in the different gut regions (oesophagus, stomach, pyloric caeca, ileum, rectum) ofH. niloticusrevealed an array of glycosidases (amylase, sucrase, maltase, lactase, cellulase); proteases (pepsin, trypsin, chymotrypsin) and lipases were also detected. The pattern of distribution and relative activity of the enzymes is consistent with its planktophagous diet. Haematocrit values, haemoglobin concentrations, erythrocyte and leucocyte counts, mean cell haemoglobin concentration, plasma protein, glucose and albumin, inorganic ion (sodium, chloride, calcium, potassium, magnesium, phosphorus) concentrations of pond-culturedH. niloticuswere established. Correlations were found between some blood parameters and standard length.