I. ricinus is the most important vector of tick borne encephalitis virus and of Babesia. The existence in the hemolymph of different I. ricinus populations of a Rickettsia sp. related to the RMST group (Rocky-Mountain spotted fever), a trypanosome close to Trypanosoma theileri and an infectious larval form (L3) of Dipetalonema rugosicauda is described. An outline is suggested to illustrate functioning of a natural foci of tick encephalitis. Biological significance of the unusual presence of trypanosomes and of larval filariae in ticks is discussed. Rickettsia, trypanosomes and filarial forms are observed for the 1st time in Swiss I. ricinus.