Abstract The aim of this work was to test the vectorial competence of A mblyomma tonelliae ( I xodida: I xodidae) to transmit R ickettsia rickettsii ( R ickettsiales: R ickettsiaceae), the agent of R ocky M ountain spotted fever ( RMSF ). All parasitic stages of A . tonelliae were exposed to R . rickettsii by allowing each stage to feed on hosts inoculated with this pathogen. Thereafter, ticks were fed on uninfected hosts. All stages of A . tonelliae were able to acquire the R . rickettsii infection and maintain it by transstadial and transovarial transmission. When infected ticks fed on uninfected hosts, the hosts developed rickettsiosis disease. This study demonstrates the vectorial competence of A . tonelliae to transmit R . rickettsii . These results have epidemiological relevance because A . tonelliae is one of the tick species most likely to infest humans in A rgentina, including in areas in which RMSF has been reported.