The amino-acid sequences corresponding to the circumsporozoite protein (CSP) of Plasmodium vivax fall into two main types, VK210 and VK247, each of which has a characteristic tandem repeat. When the repetitive domains of the CSP gene from six isolates of P. vivax from South Korea were sequenced they were found to show a total of 20 tandem amino-acid repeats, and repeat patterns that are regular and distinct from those of other P. vivax isolates. The amino-acid sequences of the South Korean parasites were found to be most similar to those of three isolates from China (CH-3, CH-4, and CH-5) and, particularly, to one from North Korea. A sequence (AGGNAANKKAEDAGGNA) and two repeats of the sequence GGNA found in the North Korean parasites were found in all six isolates from South Korea. The South Korean parasites investigated appear phylogenetically identical and unique to the Korean peninsula.