Abstract Human Kv3.3/KCNC3 is a Shaw-type potassium channel that has been mapped to chromosome 19q13.3–13.4. Complete mouse and rat Kv3.3 cDNA coding sequences have been published, yet the human Kv3.3 cDNA has remained incomplete for years. We report here for the first time the amino acid sequence for hKv3.3 and the electrophysiological behavior of the encoded channel in transiently transfected mammalian cells. In addition, we report the occurrence of Kv3.3 message in rabbit corneal endothelial cells and the properties of the currents when the corneal channel is expressed. The hKv3.3 gene is highly GC-rich (69%) and contains numerous GC runs which made DNA sequencing and PCR amplification especially problematic. The full-length sequence contains two possible start codons. The encoded 757 amino acid hKv3.3 protein is about 93% identical to mouse and rat Kv3.3 in the first 659 amino acids before the C-terminal domains diverge greatly as a result of alternative splicing. The rabbit cornea Kv3.3 is a close sequence match for hKv3.3 even in the C-terminal domain. However, we have not yet found a cornea sequence which contains the first potential start codon from hKv3.3. Electrophysiologically, the hKv3.3 channel produces an A-current although expression of constructs which lack the 5′ region of the first start codon inactivate much more slowly than full-length constructs. This short hKv3.3 construct also shows changes in activation.