Erythema chronicum migrans (ECM) developed in a man after a tick bite; the tick was probably lxodes pacificus. Despite extensive laboratory evaluation, including bacterial culture of involved skin, viral and rickettsial titers, biopsy and special stains, animal inoculation, and electron microscopy, the causal agent could not be identified. Although probably unrelated, an elevated measles titer with a twofold drop on serial determinations was of interest. To our knowledge, this report documents the first reported case of ECM acquired in this country in which the tick that caused the disease has been recovered.