When properties of extracellular fluids that might regulate the ability of the Lyme disease spirochete to locomote were investigated, the rate of progression correlated with viscoelasticity. Such spirochetes flexed and rotated but did not progress in relatively nonviscous fluids and migrated increasingly rapidly as the viscous characteristics ofthe medium increased. The viscoelastic properties of various kinds of hyaluronic acid resembled those of a methylcellulose standard. The maximum velocity that spirochetes achieved in such solutions related directly to viscoelasticity rather than to chemical composition. Spirochetes remained motile during 3 h ofobservation despite 100-fold dilution of the standard nutrient medium. The immobility of Lyme disease spirochetes in media less viscous in character than fixed tissue suggests dissemination via the intercellular ground substance of skin.