In Northumberland Dodecaceria concharum coexists in the same habitat as D. caulleryi. Morphologically both species are very similar but reproductively they are not. D. concharum is a parthenogenetic atoke while D. caulleryi has an asexual cycle that ends in an epitokous sexual phase. Histologically the gonad of both species had six cell types and the number of cells in each type were found during gametogenesis. In D. concharum the period over which gametogenesis occurs was followed from the increase in size of free coelomic oocytes and numbers of body segments. In D. caulleryi the period was followed by means of changes in the asexual cycle and metamorphosis into epitokes. In spite of the differences in the methods of reproduction of the two species gametogenesis of each was found to be almost identical. Only oogenesis could be compared since D. concharum does not have males. Prior to spawning in both species the production of oocytes by the ovary appeared to be inhibited. Possible hormonal mechanisms used to control atokous and epitokous spawning are discussed. From the histology of the gonads and oocytes D. concharum appeared to have half the number of chromosomes of D. caulleryi.