Borrelia crocidurae infection rates in different organs of adult Ornithodoros (Pavlovskyella) erraticus and O. (Ornithodoros) savignyi were studied at intervals during a 60-day period following an infective meal. The spirochaetes persisted in tick guts for 1 month, were first observed in other organs 1–7 days post-feeding, and persisted for 20–60 days in different organs. The ovaries, Gene's organ, testes and male accessory glands of only O. erraticus were infected. The infection rates in adult and immature O. erraticus and O. savignyi post-feeding on infected hamsters were 26·7–60·0% and 26·7–80·0%, respectively. Spirochaete trans-stadial transfer from originally infected larval O. erraticus to first-instar nymphs (N1) was 33·3%; to subsequent N2–N5 it was 40·0–60·0%. Infection rates in the resulting adults were 26·7% (males) and 46·7% (females). In O. savignyi, trans-stadial transfer from originally infected N1 to N2 was 26·7% and to subsequent N3–N7 13·3–33·3%; of the resulting males and females, 20·0% were infected. Transovarial transmission from female O. erraticus infected as adults occurred after the second gonotrophic cycle to 33·3–53·3% of the F1 larvae, and to 26·7 and 46·7% of the F1 males and females, respectively. All infected F1 females transmitted the spirochaetes to 26·7–73·3% of the F2 larval progeny after the first gonotrophic cycle. Transovarial transmission was not observed in O. savignyi.