Between 1954 and 1963, typhus, both epidemic and recrudescent, has been studied in Bosnia. The recrudescent typhus cases occur sporadically throughout the year, whereas epidemic typhus occurs mainly between November and June. During 1954-63, some 10-20 years after the extensive typhus epidemics of the Second World War, the highest incidence of recrudescent typhus was in the 20-50 year age-group, whereas epidemic typhus reached its highest incidence in the 2-30 year age-group. Field observations support Zinsser's hypothesis that man is the interepidemic reservoir of typhus. Recrudescent typhus is not likely to cause epidemics of primary epidemic typhus in environments where lice are scarce. However, if typhus is to be eradicated, it seems that it will be necessary to destroy all lice in areas where there are people who have at some time had the disease.