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An increase of the number of Borrelia burgdorferi s. l. in nymphs was observed 3-4 months later the moulting and during the following 4-5 months. The percent of infected ticks remained unchanged. The reproduction of spirochete was going not only at a room temperature, but also at a lower temperature, 3-5 degrees (imitation of hibernation conditions). An increase of the spirochete density was especially well marked, when the ticks were maintained under fluctuated daily conditions. We have found out the difference in the transphase transmission. Almost all larvae and nymphs infected by a bloodsucking retained spirochetes after moulting (the first transphase transmission). When the nymphs infected at larval stage were fed on an uninfected mouse, only 30% of imago ticks retained the spirochetes (the second transphase transmission).