Voltage-gated K+ channels play a critical role in regulating the excitability of neurons and muscle. In the nervous system of many species voltage-gated K+ channels are primarily represented by four subfamilies, Shaker, Shal, Shab, and Shaw. We have cloned a K+ channel in the leech Haemopis marmorata, called LK-1, using a PCR based screen that was designed to isolate any of the four defined subfamilies of voltage-gated K+ channels. LK-1 primary amino acid sequence reveals that it is a Shaker-related channel sharing the highest amino acid identity within the transmembrane domains (48-58%) to classified Shaker homologs in species both higher and lower on the evolutionary scale than leech. Phylogenetic analysis of LK-1 was performed to determine if LK-1 belongs to one of the previously defined subfamilies of voltage gated K+ channels. The placement of LK-1 in the trees generated indicates that it does not fit directly into, but is a sister subfamily to the Shaker subfamily. Haemopis marmorota LK-1 was used to screen a Hirudo medicinalis CNS enriched cDNA library, and the clone isolated shares 96% amino acid identity in the core region of the channel protein to Haemopis LK-1. LK-1 appears to be predominantly expressed in the CNS of both leech species. The high conservation of LK-1 across two species of leech suggests that this K+ channel may play a functionally conserved role in neuronal function. The cloning of LK-1 in leech provides an important tool for use in studies toward our ultimate goal of elucidating how ion channels regulate the excitability of neurons that are correlated with the control of neuronal networks, and the behaviors they mediate.