The gating of different potassium channels depends on many diverse factors. We now report a unique example of a K+ channel with a Cl- dependence. The slo-2 gene was cloned from Caenorhabditis elegans and is widely expressed in both neurons and muscles; it was highly abundant, as suggested by its high representation in the C. elegans EST database. SLO-2, like its paralogue, SLO-1, was also dependent on Ca2+. We show by site- directed mutagenesis that its requirements for both Cl- and Ca2+ are synergistic and associated with the same functional domain. SLO-2's dependence on Cl- implies that intracellular Cl- homeostasis may be important in regulating cellular excitability through this unusual K+ channel.