Mongolia is a country of Central Asia that occupies 1,564,116 km(2) and has a population of 2.7 million people. The geography of Mongolia is varied and has a continental climate. Plague, caused by the bacterium Yersinia pestis, is enzootic in wild rodent populations over large rural areas of Mongolia. Natural plague foci have occurred over 28.3% of Mongolia, and 47.1% of these foci are highly active. Highly active plague foci exist mainly in the western part of Mongolia. A total of 27% of all plague cultures were isolated from ectoparasites of 12 species of endemic mammals and 1 species of bird. Most plague cultures isolated from ectoparasites of mammals were from fleas (91.5%). The majority of cultures isolated from fleas were from marmot fleas (64.5% of all fleas). The marmot flea (Oropsylla silantiewi) is considered the primary vector of plague. Human cases of plague have been recorded in Mongolia since 1897 and more than 3000 plague cultures were isolated from natural foci. Plague foci occur between 50 degrees 00-43 degrees 00 longitude and 88 degrees 00-120 degrees 00 latitude and at altitudes between 640 and 3500 m.