We have cloned the cDNA for a squid Kvl potassium channel (SqKv1A). SqKv1A mRNA is selectively expressed in giant fiber lobe (GFL) neurons, the somata of the giant axons. Western blots detect two forms of SqKv1A in both GFL neuron and giant axon samples. Functional properties of SqKv1A currents expressed in Xenopus oocytes are very similar to macroscopic currents in GFL neurons and giant axons. Macroscopic K currents in GFL neuron cell bodies, giant axons, and in Xenopus oocytes expressing SqKv1A, activate rapidly and inactivate incompletely over a time course of several hundred ms. Oocytes injected with SqKv1A cRNA express channels of two conductance classes, estimated to be 13 and 20 pS in an internal solution containing 470 mM K. SqKv1A is thus a good candidate for the "20 pS" K channel that accounts for the majority of rapidly activating K conductance in both GFL neuron cell bodies and the giant axon.