In acute toxicity tests based on the loss of positive rheotaxis, the relationship between exposure time and bis (tri‐ n ‐butyltin) oxide concentration was determined for Salmo gairdneri and Tilapia rendalli . This relationship may be represented by two superimposed rectangular hyperbolas (X‐d) (Y‐b)=c corresponding to two poisoning mechanisms. The 24‐h EC ₅₀ is 30.8 μg/1 for trout and 53.2 μg/1 for T. rendalli . A histopathological study on S. gairdneri shows concentrations from 5.85 to 0.0117 mg TBTO/1 resulted in damage to the gill epithelium. At low concentrations a degeneration of the cornea and damage to the epithelial cells of bile ducts were also observed. Prior to a loss of positive rheotaxis and with concentrations from 0.023 to 1.17mg/l, the packed cell volume, the haemoglobin concentration and erythrocyte count increased. Determination of the mean cell volume showed swelling took place at 1.17 mg/1 but shrinkage was observed at 0.053 mg/1. These results indicate that TBTO interfered with the process of respiration. A ‘safe level’ (0.1 μg/1) is proposed for schistosomiasis control.