SUMMARYComplement‐fixing antibodies toCoxiella burnetiwere present in 23 p.c. of kangaroos collected in western Queensland, and agglutinins in 15 p.c. Red kangaroos,Megaleia rufa, showed a higher incidence of complement‐fixing antibodies (33 p.c.) than grey kangaroos,Macropus major(12 p.c.). In three areas, the incidence of complement‐fixing antibody inM. rufawas between 46 and 54 p.c.C. burnetiwas isolated in mice from the blood of oneMacropus major.Thirteen isolations were made from nearly 3,000 ornate kangaroo ticks,Amblyomma triguttatum, collected from kangaroos, goats and sheep. The titre of infectivity of infected ticks for mice was usually in the range 10−6to 10−9.Infection of kangaroos and A.triguttatumwas found over a considerable area, between 24 and 28° S. and 144 and 149° E., in western Queensland.It thus seems probable that a natural kangaroo‐tick cycle exists. The three‐host life cycle and wide host range of this tick would allow it to spread infection to domestic animals as well as kangaroos. Infestation of sheep would bring infected ticks into contact also with workers in the sheep‐shearing industry, and was probably responsible for recent outbreaks of Q fever among them.