Municipal sewage effluents contain complex mixtures of chemical and biological substances known to disrupt immunological functions in fish. Juvenile rainbow trout (Oncorhynchus mykiss) were exposed to concentrations of a primary-treated effluent for 45 d at 15°C in semi-static conditions. After the exposure period, the immune status – phagocytic activity, proliferation of B- and T-lymphocytes, and the ability to remove the pathogenic bacterium Aeromonas salmonicida were determined. In parallel, hepatic T₄ 5‘-deiodinase activity was determined. The results showed that the phagocytic response was bimodal; active phagocytosis was inhibited at the lowest concentration (0.003% v/v) but induced at higher concentrations (≥0.03% up to 3% v/v). The effluent inhibited proliferation of both T- and B-lymphocytes, but the inhibition was more severe for B-lymphocytes, suggesting that humoral immunity was more affected than cell-mediated immunity. The proportion of infected trout dropped in trout exposed to effluent compared to trout in aquarium water, suggesting that bacteria found in the effluent might have immunized fish against the pathogenic bacteria challenge. High K_m hepatic T₄ 5‘-deiodinase activity was increased at one effluent concentration and was negatively correlated with active phagocytosis and proliferation of T-lymphocytes but was positively correlated with proliferation of B-lymphocytes. Hierarchical tree analysis of the data revealed that phagocytic activity was closely associated with proliferation of T-lymphocytes while B-lymphocyte activity was more closely correlated with high K_m T₄ 5‘-deiodinase activity and the likelihood of developing an A. salmonicida infection. Municipal effluents have the potential to affect the thyroid and immune system of fish exposed for long periods at relatively low concentrations (≥3% v/v). Investigation of the immune status of feral fish populations and of the possible interplay between the immune system and thyroid status is warranted.