The immature oocyte and unfertilized eggs of Urechis caupo , a marine echiuroid worm, contain many nuclear bodies and a main nucleolus. These bodies are similar to prenucleolar bodies of other materials with respect to their origin, structure, cytochemical and biochemical characteristics. They are formed in large numbers during the diffuse diplotene stage, perhaps by an aggregation of nucleolar materials which had been associated with chromosomes. They are fibrillar in structure, made up mainly of some nucleolar proteins, and are inactive in RNA synthesis. Biochemical analyses suggest the possibility that these structures contain a small amount of ribosomal RNA precursors which may have originated from the main nucleolus. An accumulation of prenucleolar bodies in Urechis oocytes may result from the lack of formation of multiple functional nucleoli, such as those found in amphibian oocytes.