<p>Pesticides are commonly used all over the world to control unwanted pests in agriculture but adversely affect the non-target organisms. The widespread use of pesticides contaminates the aquatic habitat and causes fish mortality. The effect of triazophos on fish immunity has scarcely been investigated. This study was carried out to assess the triazophos-induced immune-modulation in <i>Channa punctatus</i>. The fish were kept in triazophos mixed water (5% and 10% of LC₅₀ value) for 10 and 20 days. Control fish were kept in triazophos-free water. The leucocytes were isolated and nitrite release, neutrophil phagocytosis and leucocyte respiratory burst were measured. The lymphocytes were separated through density gradient centrifugation and the cell-mediated immunity was evaluated through the multiplication of lymphocytes in the presence of the mitogens. The results of in vivo study revealed that the immune responses of the leucocytes were affected differentially after exposure to triazophos. The phagocytosis and the superoxide production were suppressed at both doses of triazophos but nitrite production was suppressed only when triazophos was used at 10% of LC₅₀ for 20 days. Further, dose and duration-dependent reduction was evident when lymphocyte proliferations were assayed. It was concluded that pesticide residues in water adversely affect fish immunity which may be the reason of the increased mortality observed in the ponds situated near the agricultural lands. The results of this study demonstrate that triazophos exposure to fish differentially suppresses the immune functions.</p>