In water of various hardness (0.24 mM Ca, 2.5 mM Ca and 3.8 mM Ca) three experimental groups were simultaneously set up: water, water + acetone (0.1%), and water + acetone + lindane 1 mg x l super(-1) (1 ppm). Both soft and very soft waters and acetone clearly induced a decrease in the animals longevity. Conversely 1 ppm of lindane increased their expectation of life. The equation of von Bertalanffy always described shell growth relative to acetone control: the value of the parameter k was decreased by 18% in very hard water but only by 7% in soft water and was even increased by 8% in very soft water; the parameter T infinity was reduced by intoxication only in hard and very hard waters. With 1 ppm of lindane a stronger mineralization of the shell was always observed. The fecundity was reduced by 1 ppm of lindane in all cases. Study of fecundity related to size revealed a direct depressor effect of this insecticide on the reproduction of Lymnaea . These results showed that the toxic effect of 1 ppm concentration of lindane was related to the calcium concentration of water.