In trematode-infected Lymnaea truncatula, as in other lymnaeids, hemocytes are formed in the connective tissue. Mitoses are found singly in blood vessels or connective tissue or occur in hemocyte nodules, developing along the mantle epithelium or associated with blood sinuses. The so-called hemocyte-producing organ in L. truncatula is not equivalent to that in Biomphalaria glabrata, but rather involves the proximal part of the kidney sac. It has a dual structure: the main part, containing podocytes and broad hemal spaces, is adapted for hemolymph filtration; the apical portion, adhering to the mantle and pericardial epithelia, has a thicker connective-tissue frame in which hemocyte nodules may develop. The role of this region of the kidney in hemocyte formation is discussed.