The fine structure of the neoblasts in the naidOphidonais serpentina has been examined. The neoblasts of control worms have a relatively large nucleus, containing a large nucleolus, a sparse amount of rough endoplasmic reticulum, and an abundance of free ribosomes and mitochondria. Although Golgi membranes have been demonstrated, there is no evidence that the neoblasts are secretory in nature. Neoblasts form loose cell-to-cell contacts with one another and with peritoneal cells. In worms 12 hours after posterior transection, the neoblasts found at the end of the severed ventral nerve cord have rounded up and are no longer spindle-shaped. Counts of neoblasts immediately after posterior transection indicate that they are equally distributed in the last five segments. A statistical analysis of their distribution during posterior regeneration reveals a significant increase in neoblasts in the last three segments and a migration of neoblasts toward the wound. The role of neoblasts in oligochaete posterior regeneration is discussed.