A Rickettsia-like infectious agent mildly pathogenic for guinea-pigs was recovered from A. maculatum collected near Cleveland, Texas. As observed in [male] guinea-pigs, there is usually a characteristic temp. curve and edema and reddening of the scrotum, but infection is sometimes inapparent. There is complete cross immunity between this infection and both Rocky Mountain spotted fever and boutonneuse fever, but with endemic typhus the degree of cross immunity is less consistent. However, it agrees with none of these diseases in all particulars.