The influence of some environmental parameters, i.e. dissolved oxygen, water temperature, Ca concentration, pH, conductibility, on populations of the river bullhead, Cottus gobio, in Northern Apennines (Tuscany) has been considered. The bullhead seemed to prefer waters with low levels of calcium. The Discriminant Function Analysis (DFA) showed congruent results and highlighted the role of calcium concentration and pH levels in determining suitable habitats for C. gobio (overall correct classification: 84.1%). The influence of calcium concentration in the water is not only critical in the definition of the presence/absence pattern, but clearly had a negative effect on the growth of C. gobio. The distribution pattern linked to the geological substrate composition of the studied area was confirmed by the distribuition of the C. gobio in two other areas in Northern Italy (the Orobie and the Maritime Alps).