Lyme disease spirochetes (Borrelia burgdorferi) multiplied rapidly in larval Ixodes dammini, reaching a mean density of 2,735 spirochetes/tick on day 15 post-repletion. A 5-fold drop in spirochete levels occurred during the subsequent premolting period. Recently molted nymphs contained a mean of <300 spirochetes/tick. Following nymphal repletion, spirochete multiplication renewed, reaching a mean abundance of 61,275 spirochetes/nymph on day 75 post-repletion. A 10-fold drop in spirochete abundance occurred again when ticks molted to the adult stage. Tick-derived spirochetes proved to be infectious when >104 spirochetes were inoculated ip into hamsters (4 of 4 animals infected). Inocula of 103–4 spirochetes were not always infectious (8 of 23 animals infected), and inocula of <103 spirochetes were insufficient for establishing infection (0 of 8 animals infected).