There is considerable interest in making a public option available in the Affordable Care Act (ACA) nongroup marketplaces. A public option could catalyze competition in less competitive markets, leading other marketplace insurers to lower their premiums. We analyze commercial insurers’ experiences competing in marketplaces with managed-care organizations, which, prior to ACA implementation, only offered coverage within public programs (either Medicaid only or Medicaid and Medicare). These managed-care organizations, though not purely public options, represent a proxy for them. Our analysis finds an association between the presence of a Medicaid insurer in a rating region and competing insurers (e.g., Blue Cross Blue Shield, provider-sponsored plans, national and local commercial insurers) offering lower premiums.