Starting from 1978, noncontagious febrile diseases of unclear etiology, accompanied by pronounced headache, roseolous-papular eruptions, prolonged convalescence period, are registered in May-September in Astrakhan Province. These diseases can be effectively treated with chrolamphenicol. In 11 out of 12 sera obtained from such patients the complement fixation test with the antigens of rickettsiae causing tick-borne spotted fever, epidemic typhus, as well as Coxiella burnetii antigen, revealed the presence of antibodies (in 8 sera) only to the antigens of rickettsiae causing tick-borne spotted fever (R. akari, R. conorii, R. sibirica), or the titers of antibodies to these antigens were greater (1 serum), equal and lower (2 sera) in comparison with those of the antigens of rickettsiae causing epidemic typhus. The dynamics and values of antibody titers in 7 patients with the antigens of three rickettsial species of the tick-transmitted biotype indicated that the disease was related to tick-borne spotted fever.