Methylcyclopentadienyl manganese tricarbonyl (MMT) is an organic derivative of manganese (Mn) used as an additive in unleaded gasoline. The combustion of MMT leads to the formation of oxides of manganese. The objective of the present study is to predict the environmental levels of Mn and the human exposure in the St-Lawrence ecozone (fluvial section, Quebec, Canada) using an environmental fate/exposure model: GEOTOX. The results of our MMT research program on abiotic and biotic components of the ecosystem and on the human exposure were used to validate the model estimations. Air and surface soil were selected as source terms with an annual Mn input rate in each compartment of 0.083–0.113 mol km−2d−1and 0.44–0.87 mol km−2d−1respectively (Mn3O4equivalent). The predicted air, soil, plant, surface water and sediment concentrations were similar (± 50%) to values measured in the Montreal region. As expected, the ingestion pathway was the main absorption route for adults (> 99%), with vegetables and fruits contributing almost 80% of the dietary intake of Mn. The multimedia exposure doses for adult men predicted by the model ranged between 0.04 and 0.08 mg kg−1d−1compared to 0.004 and 0.201 mg kg−1d−1(average = 0.05) for workers from the MMT study. Considering the landscape configuration and the source vectors (air and soil) included in the model, GEOTOX estimations were in good agreement with measured values.