The proneural genes of achaete-scute (ac-sc) family that encodes the bHLH class transcription factors play a variety of roles in neurogenesis. In Hydra, the ac-sc homologue CnASH is involved in nematocyte differentiation. In the present study, we found that sensory neurons in the tentacles expressed CnASH, in addition to differentiating nematocytes in the body column of Hydra. Neuron precursors that migrated to the tentacle base did not express CnASH, and it took 1 day for them to become CnASH-expressing neurons. Thus, the CnASH-positive cells at the tentacle base appeared to be sensory cells at early stages of differentiation. Furthermore, the CnASH-positive neurons distributed from the base to the tip of tentacles suggest that the gene is also involved in maintenance of the differentiated state. In addition, we found that the sensory neurons in the tentacles consist of at least two subpopulations. The comparison of the CnASH expression with Nv1 expression in sensory cells that is detected by monoclonal antibody Nv1 showed that at least Nv1-positive/ CnASH-positive and Nv1-negative/ CnASH-positive sensory neurons existed in the tentacles.