Previous studies onArenicola marinasuggested that oocyte maturation was induced by a single maturation hormone from the prostomium. This maturation hormone was thought to act directly on the oocyte (Meijer and Durchon, 1977), A recently described species,Arenicola defodiens(Cadman and Nelson-Smith, 1993), morphologically very similar toA. marina, has been found at the sampling sites described by Meijer and Durchon (1977). Results presented here from studies on British populations ofArenicola marinashow that in this species, oocyte maturation is controlled by two hormonal steps. The first step involves the prostomial maturation hormone. The second step depends on a maturation inducing substance in the coelomic fluid. We will refer to this as the coelomic maturation factor (CMF). A reliablein vitroassay for oocyte maturation in the lugwormArenicola marinahas been adopted. It utilizes fluorescence staining of the chromosome material with DNA labelling dyes (Hoechst 33342 and 33258). Maturation of oocytes inA. marinainvolves germinal vesicle breakdown (GVBD). This is accompanied by the movement of chromosomes from late prophase to metaphase of meiosis I and chromosome condensation. The chromosomes are stained brightly by the dyes and their relative positions can be easily identified so that mature and immature eggs can be distinguished by the differences in chromosome position and form. The development of thein vitrofluorescence assay has enabled us to demonstrate that there are two endocrine steps involved in the induction of oocyte maturation. We have begun the characterization of CMF, and data show this to be a thermolabile molecule with a molecular mass greater than 10 kd.