In the 20 years since the first agent of Lyme disease was discovered, much interest has focused on the possible biological roles of a few outer surface proteins (Osps) in the alternating life cycle that includes ticks and vertebrate hosts. Two major proteins, OspA and OspC, are differentially regulated by the spirochaete Borrelia burgdorferi during the several days when ticks feed. The reciprocal decrease in OspA with the rapid up-regulation of OspC by the spirochaetes when ticks are feeding suggests that OspA aids in spirochaete attachment while OspC assists in the dissemination of spirochaetes from tick to vertebrate. Future experiments in ticks with mutant spirochaetes that lack these proteins should clarify the speculative functions currently given to these proteins.