In the urine of plague-infected great gerbils Yersinia pestis capsular antigen was detected by means of diagnostic preparations, both commercial and experimental (based on monoclonal antibodies). The antigen was detected in many urine samples taken from the animals over a prolonged period. The incidence and duration of antigenuria were found to be related to the survival time of great gerbils after infection and the level of antibodies in their blood. The number of animals with antigenuria markedly exceeded the number of animals from which Y. pestis was isolated, especially at a later period after infection. Examinations of urine samples from live great gerbils trapped in natural foci appears to be a method more effective in epizootiological survey than the bacteriological analysis of the animals.