The appearance and localization of acetylcholinesterase (AchE) were studied during embryonic development of the leech Helobdella triserialis. Use of a histochemical stain showed that AchE is present in the polar plasms of the uncleaved leech egg. in the course of the first two cleavages, the AchE is transmitted along with the polar plasms mainly to the D blastomere, precursor of mesoderm and ectoderm. At these early developmental stages, AchE also lines the cleavage furrows between the blastomeres. At later stages, AchE is present in the parental meso- and ectoteloblasts and their daughter stem cells that form the germinal bands. At the completion of germinal band coalescence, AchE is present in the rostral and caudal regions of the germinal plate. Thus, the presence of AchE in the early phase of embryogenesis precedes the formation of any nervous tissue. In the late phase of embryogenesis, once the embryo has reached the stage of body closure and a nervous system is present, the distribution of AchE is that characteristic of the adult leech: AchE is localized in the neuropil of the segmental ganglia of the ventral nerve cord and in the musculature of the body wall. Therefore, at this still quite immature stage, the embryonic nervous system has taken on not only the morphological but also some of the neurochemical characteristics of the adult leech.