The amoebocyte-producing organ in Lymnaea truncatula is located between the pericardium and the kidney. In control snails bred at 20 degrees C, the organ only has fibroblast-like cells and type I amoebocytes; its growth is associated with a decrease in cellular density. The organ in infected snails bred at 20 degrees enlarges from day 1 to day 21 after exposure (phase 1) with a rapid increase in the number of type I amoebocytes. Then a gradual cellular rarefaction occurs in this organ from day 21 to day 70 after exposure (phase 2) with a fall in the number of type I amoebocytes and a correlated increase in the number of type II amoebocytes. The connective septae become "fibrous" with thickening from day 42 to day 70 after exposure. The breeding temperature only affects the timing and the dates when the different developmental phases begin in this organ. At a given temperature, the body volume of the snail affects only the cellular density. The amoebocytic rarefaction in the organ is uninterrupted if the snail begins its fast on day 49 after exposure (phase 2, 20 degrees C); the organ becomes "fibrous" if the fast begins on day 14 after exposure (phase 1, 20 degrees C). The significance of these observations is discussed.