Chicken manure is added to freshwater fish ponds as pond fertilizer and supplementary feed in Hong Kong as well as in other Asian and East European countries. The present study was undertaken to observe the morphological changes in the gills of tilapia treated with chicken manure using scanning and transmission electron microscopy. Tilapia, Sarotherodon mossambicus, were fed chicken manure as a supplementary diet for four weeks under laboratory conditions. This produced a swelling of the gill epithelium and enlargement of the sub-epithelial space in nearly all the lamellae of the fish gill. Cell junctions, i.e., desmosomes, were disrupted with separation of adjacent epithelial cells in the severely damaged fish gills leading to the exposure of the underlying basement membrane and the capillary endothelium to the water. Such damage of the wate—blood barrier of the fish gill will impair the respiratory function of the fish. Copyright © 1987 Published by Elsevier B.V.