Effect of water temperature, feeding levels and stocking densities on the immune response of Nile tilapia, Oreochromis niloticus, vaccinated with Edwardsiella tarda crude lipopolysaccharide (LPS) was determined in laboratory controlled experiments. 0. niloticus reared under optimal environmental factors and those exposed to other factors, with exception of low water temperature, responded to immersion (DP) vaccination in crude LPS by production of high levels of secreted antibody in the body surface mucus pronounced from first week post-vaccination (PV), 5 by Log2, and reached 10:11 at 10 week PV. By contrast, O. niloticus exposed to low water temperature (16 + 1C) had low levels of secreated antibody in the body surface mucus (2 at 1 week and 6 at 10 week PV).