A study of the density of river mollusks in six watercress groves is made. It is seen how densities of F. cubensis, intermediate host of fascioliasis in Cuba, is maintained with relative stability. The poor effectiveness of P. duryi as an agent of biologic control is demonstrated. The high density of P. cubensis, which seems to exert a certain regulatory effect on F. cubensis--particularly in the months of reproductive peaks (August and September)--is emphasized. Densities of B. havanensis and B. helophila may be affected by the density of F. cubensis and P. cubensis, which may exert certain influence in the decrease of pulmonate density. F. cubensis and P. parvulus are morphometrically compared and a high correlation between length and width of the shell (r = 0.9) is emphasized; such correlation was smaller in P. parvulus (r = 0.7).