In regulär monthly samples from a population of Harmothoe imbricata, between 8 and 35% of the individuals were found to be regenerating lost posterior Segments. Breakage always occurred cleanly at an intersegmental boundary and the body wall at this point is specialized for autotomy. The septa are inserted just anterior to the plane of autotomy in each segment. The septa are displaced from the transverse plane in H. imbricata by the insertions of two oblique muscles which have become functionally intersegmental. Neither starva- tion nor removal of the brain influences the rate at which posterior segments are regenerated after experimental ablation.