Calcium-activated potassium channels regulate excitability of the adult nervous system. In contrast, little is known about the contribution of calcium-activated potassium channels to excitability of the embryonic nervous system, when electrical membrane properties and intracellular calcium levels show dramatic changes. Embryonic Xenopus spinal neurons exhibit a well-characterized developmental program of excitability that involves several different currents including calcium-activated ones. Here, we show that a molecular determinant of calcium-activated potassium channels, xSlo, is expressed during Xenopus embryogenesis, even prior to differentiation of excitable tissues. Five different xSlo variants are expressed in embryonic tissues as a consequence of alternative exon usage at a single splice site. One of these variants, xSlo59, is neural-specific and its expression is limited to late stages of neuronal differentiation. However, expression of the 4 other variants occurs in both muscle and neurons at all stages of development examined. Electrophysiological analysis of recombinant xSlo channels reveals that the xSlo59 exon serves as a gain-of-function module and allows physiologically relevant levels of membrane potential and intracellular calcium to activate effectively the resultant channel. These results suggest that xSlo59 channels play a unique role in sculpting the excitable membrane properties of Xenopus spinal neurons.