Hispid cotton rats (Sigmodon hispidus Say and Ord) were susceptible to infection with Rickettsia rickettsii Wolbach under laboratory conditions and were capable of serving as sources for infecting ticks with rickettsiae. Cotton rats developed rickettsemias that could be detected for as long as 6 h following intraperitoneal inoculation of 10(5) plaque-forming units (PFUs) of R. rickettsii (Morgan strain). An estimate of the median infectious dose (ID50) was made by inoculating six groups (eight animals per group) with serial dilutions of 10(1) to 10(6) PFUs. In this experiment, cotton rats became infected after exposure to relatively few organisms (ID50 = 37 PFUs). None of the 48 cotton rats succumbed to infection, and only 6 of the 48 rats failed to seroconvert. Dermacentor variabilis (Say) larvae were fed on rickettsemic cotton rats to determine whether this species could serve as a source for infecting ticks with R. rickettsii. A small percentage (0.9-3.0%) of larval ticks that fed on three of the four cotton rats had R. rickettsii in their tissues when examined as nymphs. A fourth cotton rat died 7 d after inoculation with R. rickettsii and infected a much higher percentage of ticks (64.0%) than the other three animals.