In the geographically parthenogenetic mayfly, Ephoron shigae, egg maturation and counts of chromosome number of unfertilized, parthenogenetic eggs were studied, in comparison with fertilized eggs from a bisexual population. The primary oocyte becomes mature through two successive maturation divisions. The first maturation division (meiotic division) takes place in the primary oocyte to produce a secondary oocyte and a first polar body. The second maturation division soon occurs in the secondary oocyte, in which the nucleus is divided into a mature egg nucleus (female pronucleus) and second polar body nuclei. The first polar body, in some cases, was successively divided into two polar bodies; in other instances, it was not divided. After the successive maturation division, the egg nucleus and the sister second polar body nucleus drew near to fuse into the zygote nucleus. The chromosome number was doubled in the zygote, and this conjugation initiates subsequent embryonic development. This suggests that, in E. shigae, the process of parthenogenetic recovery of diploidy is the automictic type categorized as the ‘terminal fusion’ pattern. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99, 335–343.