The motility of bacteria is an important factor in their infectivity. In this study, the motility ofLeptospira, a member of the spirochete family that causes a zoonotic disease known as leptospirosis, was analyzed in different viscous or osmotic conditions. Motility assays revealed that both pathogenic and saprophytic strains increase their swimming speeds with increasing viscosity. However, only pathogenicLeptospira interrogansmaintained vigorous motility near physiological osmotic conditions. This suggests that active motility in physiological conditions is advantageous whenLeptospiraenters hosts and when it migrates toward target tissues.