The cytology of a diploid amphimictic (2x=26) and a triploid parthenogenetic (3x=39) cytotype of Lumbricillus lineatus has been described. The triploid type never produces mature sperm, but it must be fertilized by sperm from the diploid cytotype in order to produce viable eggs. However, the sperm does not enter the egg, but eggs which have not been activated by sperm die at an early stage. Oogenesis in the triploid cytotype, which results ultimately in the restoration of the somatic chromosome number, follows a pattern which has not been described in other parthenogenetic organisms. The first meiotic division is asynaptic and no typical metaphase plate is formed; the univalents are distributed to opposite poles without undergoing an equational division. The first division is therefore numerically approximately reductional. The nuclei are at anaphase when the eggs leave the animal, whereas the nuclei of the diploid amphimictic cytotype are in MI when the eggs are laid. Following activation by spermatozoa, the first anaphase spindle is much elongated, and becomes V-shaped. The first anaphase spindle persists and no second anaphase spindle is formed. At the second division the chromosomes divide mitotically. The resulting four chromosome groups fuse two by two in such a way that two nuclei are formed each with the full somatic (triploid) chromosome set (cf. Figs. 14–16). The different steps involved in the evolution of this mechanism are discussed in relation to the evolution of the triploid cytotype.