The squid giant axon is a classical model system for understanding both excitable membranes and ion transport. To date, a Na⁺-driven Cl-HCO₃ exchanger, sqNDCBE--related to the SLC4 superfamily and cloned from giant fiber lobe (GFL) cDNA--is the only HCO₃-transporting protein cloned and characterized from a squid. The goal of our study was to clone and characterize another SLC4-like cDNA. We used degenerate PCR to obtain a partial cDNA clone (squid fiber clone 3, SF3), which we extended in both the 5' and 3' directions to obtain the full-length open-reading frame. The predicted amino-acid sequence of SF3 is similar to sqNDCBE, and a phylogenetic analysis of the membrane domains indicates that SF3 clusters with electroneutral Na⁺-coupled SLC4 transporters. However, when we measure intracellular pH (pH_i) and membrane potential--or use two-electrode voltage clamping to measure currents--on Xenopus oocytes expressing SF3, the oocytes exhibit the characteristics of an electrogenic Na/HCO₃ cotransporter, NBCe. That is, exposure to extracellular CO₂/HCO₃^- not only causes a fall in pH_i followed by a robust recovery, but also causes a rapid hyperpolarization. The current-voltage relationship is also characteristic of an electrogenic NBC. The pH_i recovery and current require HCO₃^- and Na⁺, and are blocked by DIDS. Furthermore, neither K⁺ nor Li⁺ can fully replace Na⁺ in supporting the pH_i recovery. Extracellular Cl^- is not necessary for the transporter to operate. Therefore, SF3 is an NBCe, representing the first NBCe characterized from an invertebrate.