The effects of digested sludge on the ultrastructure of gills of Sarotherodon mossambicus were investigated. Samples of digested sludge were collected from the Shatin Sewage Treatment Plant in Hong Kong and they were (1) sun-dried (NS) or (2) sterilized in an autoclave (SS). They were then used as supplementary fish feed to cultivate the freshwater tilapia, S. mossambicus, for 50 days under laboratory conditions. The SS at low dosage (25%) had the lowest toxicity among different treatments. A swelling of lamellar epithelium, the enlargement of the the subepithelial space, the collapse of capillaries, and the infiltration of polymorphonuclear cells in the lamellae of fish gills were common to fish fed 50 to 100% NS and SS. The thickening of the basal lamina in the gill lamellae is a common feature found in the sludge-treated fish.