Abstract Mehlis' gland of both Diplozoon paradoxum and Calicotyle kroyeri is composed of two cell types that taper to form ducts opening into the lumen of the ootype. The cells are invested with fibrous interstitial material and form a close structural relationship with surrounding parenchyma. The most prevalent cell type, the S 1 cell, is characterized by an extensive GER with narrow cisternae containing a finely granular material, and numerous Golgi stacks involved in the formation of multi-vesicular secretory bodies. In the S 2 cells the GER cisternae are greatly distended with a finely filamentous material and the Golgi give rise to dense secretory bodies with a packed fibrous appearance. There are species differences in the fine structure of the secretory bodies and these may reflect differences in the chemical composition of the glands. The ducts of the glands are lined with microtubules and are anchored to the ootype epithelium by septate desmosomes. They convey the secretory products to the ootype where they are released, apparently by exocytosis involving membrane fusion, into the lumen. The ootype is lined by a highly folded cellular epithelium which in D. paradoxum is ciliated. The cells contain profiles of GER and Golgi complexes and produce a third type of secretion which is also discharged into the ootype lumen.