AbstractThe spatial and seasonal distribution of the viable heterotrophic population and three heterotrophic physiological groups, i.e. ammonifyers, nitrate reducers and sulphate reducers, were studied in two reservoirs in the province of Vizcaya (Spain). Using indicative parameters, both reservoirs were classified as warm monomictics, one hypereutrophic and the other mesotrophic. A direct relation between the size of the bacterial populations and the trophic levels of the reservoirs was observed. Likewise, the distributions of the bacterial populations in the water column are basically different. In the hypereutrophic lake there is a direct relationship between the bacterial distribution and the physical‐chemical environment of the reservoir that cannot be observed in the mesotrophic lak