Peromyscus maniculatus subjected to bot fly (Cuterebra) parasitism in the laboratory showed significant changes in amount and temporal distribution of several activity patterns. Strenuous activities, such as running in an exercise wheel or stereotypic somersaulting, declined while less strenuous activities associated with maintenance behaviors increased. Diurnal response to handling and disturbance declined significantly in infected mice. The temporal distribution of nocturnal activity was altered, and may affect vulnerability of infected mice to small predators. Most changes persisted for at least a week postinfection.