An outbreak of foot-and-mouth disease in a partially immune population of cattle in Botswana is described. The results show that when cattle immunized by vaccination were presented with natural field challenge of FMD, many animals with immunity sufficient to protect them against clinical disease were, however, susceptible to pharyngeal infection and subsequently became virus carriers. The proportion of animals becoming carriers appeared to vary with the degree of severity of the challenge.Vaccination before exposure to virus appeared to have little effect on the duration of the carrier state. No evidence was obtained of the spread of carrier virus to immune herds following the outbreak.Antibody titres during the outbreak were higher in the clinically infected animals than in the carrier animals and the uninfected animals. Evidence suggested that natural challenge boosted the titres of immune animals. After the outbreak, however, it was not possible to distinguish by their antibody titres between the carrier animal and the virus-negative animal.Antigenic studies on the strains of virus isolated are described.