Private insurers including Centene Corp., Molina Healthcare and Anthem Inc, offer individual Obamacare plans, though it is a small portion of their overall business.
The Texas court decision late on Friday pushed down shares of those companies as investors took account of the new risk to insurers who provide individual insurance and Medicaid coverage.
The ACA is widely considered to be the signature domestic achievement of former President Barack Obama.
Republicans in Congress failed to overturn it through legislation, despite being pushed to do so by U.S. President Donald Trump. But the administration has been able to take away some of its provisions and introduced new rules to bolster competing plans. It has also cut advertising and services to promote the Obamacare plans.
The new plans are attractive to individuals who do not qualify for income-based government subsidies that make Obamacare plans more affordable, Leerink analyst Ana Gupte said in a recent note.