We have isolated in BSK medium a spirochete of <i>Ixodes ricinus</i> ticks. The ticks were collected from an area of Switzerland where erythema chronicum migrans, a tick-borne, penicillin-ameliorated inflammatory disorder, is endemic. The <i>I. ricinus</i> spirochete was very similar in its morphology, polyacrylamide-gel-electrophoresis profile, and antigenic determinants to a spirochete that was previously isolated from <i>Ixodes dammini</i> ticks of the United States.
