Relative abundance and richness in molluscan species have been studied along 91 stations in 9 different streams. The fact that a significative relationship exists between several abiotic parameters and the presence of certain molluscan species is demonstrated by a factor analysis (correspondence analysis). High values of oxydability (> 3 mg/l), ammonium ions (> 0.5 mg/l), nitrites (> 0.3 mg/l), phosphorus (> 0.5 mg/l) and low oxygen content of the water (≤ 5 mg/l) exert an inhibitive effect on the species distribution (figs. 3-4). Eighteen physical and chemical factors are classified according to the effect they exert on the general distribution of molluscs by their contribution to the F1, F2, and F3 axes (table II). The pattern of sensitiveness to pollution of molluscan species is correlated with a biotic index (fig. 5, table III).