Cercariae of Psilotrema oligoon and Notocotylus attenuatus encyst free to form multilayered, hemispherical cysts. The former consists of four layers in which the innermost (layer 4) is divided into two, chemically and structurally different, areas. Cysts of the latter have three layers in which the innermost (layer 3) is again divided into two chemically different parts. A third cyst, produced by the encystment of Cercaria tarda in the tissues of Limnophilus flavicornis, is spherical and contains three layers (of which the outermost was not studied).All of the cysts studied contain a high proportion of protein, which in some layers occurs combined with carbohydrates. In both free-encysting species the two outer layers are produced by expansion of a non-cellular layer covering the integument and the innermost layer is produced by secretion of bâtonnets from a layer of cells lying beneath the body wall. Cysts of P. oligoon are divided into two separate parts, each having two layers, whereas those of N. attenuates consist of a single three-layered wall. The innermost layer in both consists of a protein with disulphide and sulphydryl linkages, and in the cyst of P. oligoon it has a circular ventral area containing carbohydrates. A similar carbohydrate-containing area occurs in the innermost layer of the cyst of N. attenuatus but as a ventro-lateral ring of material. The cyst of Cercaria tarda is also predominantly proteinaceous and stains intensely for RNA.