Acid-sensing ion channels (ASICs) constitute a family of neuron-specific voltage-insensitive sodium channels gated by extracellular protons. Functions of ASICs in mammals include nociception, mechanosensation, and modulation of synaptic transmission. However, the role protons play in mediating the effects of ASICs remains elusive. We have examined ASICs from the ascidian Ciona intestinalis, a simple chordate organism whose nervous system in the larval stage exhibits high similarity to that of higher vertebrates. We found that the ascidian genome contains a single ASIC gene that gives rise to two splice forms analogous to the mammalian ASIC1 and ASIC2. CiASIC is expressed in most neurons of the larva but is absent in the adult. Despite high sequence similarity with mammalian counterparts, CiASIC is proton-insensitive when examined in heterologous systems or in larval neurons; the latter rules out the possibility that proton sensitivity is conferred by accessory proteins or particular factors present only in Ciona neurons. Down-regulation of the CiASIC transcript by double-stranded RNA disrupted the regular pattern of larval swimming, implying that proton-independent mechanisms mediate the effects of ASIC in vivo. Together the data identify ASIC as a highly conserved channel distinctive of chordate nervous systems and show that protons are not essential for ASIC function. © FASEB.