Since Rocky Mountain spotted fever (RMSF) was recognized in Costa Rica, ecological studies have been undertaken to discern its distribution and the natural cycle of Rickettsia rickettsii. The population of ticks parasitizing animals and vegetation was scarce. Rickettsiae were not isolated from pools of ticks nor from spleens of the animals studied. However, the active circulation of rickettsiae was evidenced through the demonstration of spotted fever group antibodies in humans, dogs, and wild rabbits. These findings concur with those in a number of fatal human cases as well as with the isolation of R. rickettsii from wild rabbit ticks.