Ectoparasite faunas of 270 Rattus norvegicus and 378 R. rattus were studied. These rodents were taken alive from untreated rural premises in areas of four southern states where the average noon relative humidity in July is from 50 to 60 per cent. Collections were made during a period when ectoparasites and incidence of typhus were at high levels. The monthly abundance of the Oriental rat flea, Xenopsylla cheopis, was found to be associated with the monthly incidences of typhus antibodies in both Norway and roof rats. Populations of the tropical rat mite showed no significant association with the monthly incidences of antibodies in rats. Populations of the spined rat louse were not found to be related significantly to the occurrence of typhus in rats, as indicated by presence of antibodies, and is considered of no practical importance in natural transmission among the rats considered in this study.