The oocyte nuclei of Bithynia tentaculata have been investigated by light and electron microscopy. The small oocyte nuclei of the pachytene stage are characterised by a well developed “bouquet” arrangement of the chromosomes, a small nucleolus located at the nuclear envelope and lamellar inclusions of unknown function. During previtellogenesis, oocytes contain typical molluscan amphinucleoli consisting of the nucleolus and the so called “Proteinkorper”. It is suggested, that the Proteinkorper is identical with the “Binnenkorper” of insects. Living oocyte nuclei of Bithynia have been isolated and investigated by phase contrast microscopy. They contain lampbrush chromosomes bearing chromomeres, pairs of lateral loops and some spheres. Maximal loop formation occurs early in oogenesis whereas chromosomes of later developmental stages show contraction of the loops and formation of many spheres. Their histochemistry and ultrastructure is described.