38 late nymphs of O. turicata which had engorged on a guinea-pig ill with American Q fever (Nine-Mile fever) were tested for transmission of R. diaporica and for the presence of this organism in the tissues. The earliest and latest tests by feeding were 7 weeks and 20 mos., respectively. A total of 88 feedings failed to infect guinea-pigs. 29 ticks survived for tests by injn.; 22 of these produced typical infections, and 7 failed. The earliest and latest "positive" injns. were at 0 and 1,001 days, and the earliest and latest "negative" injns. were at 82 and 900 days, respectively. Progeny of these ticks failed to infect the test guinea-pigs when tested as follows: 2,179 larvae by feeding, 995 by injn.; 781 nymphs by feeding, 214 by injn. Tick excrement proved infective. The virulence of R. diaporica was not affected by its long residence in the tick nor by prolonged fasting of the tick.