Within the Caenorhabditis elegans genome there exist at least 42 genes encoding TWK (two-P domain K(+)) channels, potassium channel subunits that contain two pore regions and four transmembrane domains. We now report the first functional characterization of a TWK channel from C. elegans. Although potassium channels have been reported to be activated by a variety of factors, TWK-18 currents increase dramatically with increases in temperature. Two mutant alleles of the twk-18 gene confer uncoordinated movement and paralysis in C. elegans. Expression of wild-type and mutant TWK-18 channels in Xenopus oocytes showed that mutant channels express much larger potassium currents than wild-type channels. Promoter-green fluorescent protein fusion experiments indicate that TWK-18 is expressed in body wall muscle. Our genetic and physiological data suggest that the movement defects observed in mutant twk-18 animals may be explained by an increased activity of the mutant TWK-18 channels.